Category: Construction

How to build aquarium elements, decor and equipment

Book Review: Aquatic Systems Engineering: Devices and How They Function

This is a review of the new CAS library book, Aquatic Systems Engineering: Devices and How They Function, by P.R.Escobal. The author is an aeronautics engineer that left the field to form the companies Aquatronics and Filtronics, manufacturers of “high-end” aquarium filtration, aeration, and sterilization equipment.

The book discusses the use of these items in setting up an aquatic environment: both single tank and multi-tank systems. The proper sizing of components is discussed in detail, and mathematical rigor is used. This book is therefore not light reading: in fact, tables, schematic diagrams, and calculation make up the bulk of the book.

The calculations are however complicated by the exclusive use of American Imperial units throughout the book, rather than the much simpler SI units. As the author is American, this is understandable, but even American engineers will nowadays calculate in SI and (if required) convert the result to antique units after it is found.

After an introductory chapter that defines the terms used throughout the book, the meat of the book begins with a surprisingly complicated discussion on how to determine the time required for passing all of an aquarium’s water through a device (such as a filter). The complication arises from the fact that the water from the filter is mixed with unfiltered water on return to the aquarium. It is shown that all of the water can never be filtered in such a set up, but 99.99% of it is effectively filtered after the volume of water is cycled through 9.2 times. One can safely round this to 10 times, and say, for example, that a 100-liter per hour filter would require 10 hours to filter all the water in a 100-liter tank. The commonly used aquarist “rule of thumb” is to use a filter that pumps the aquarium’s water volume two to three times per hour, reducing the filter time to about four hours.

The next chapters deal with the proper sizing and operation of ultraviolet sterilizers. It is here that the author shows his biases. Chapter three opens with the sentence “Fishwise, the single most important device available today, ranking second only to a well designed mechanical filter system is the ultraviolet sterilizer”. Even setting aside this statement’s self-contradiction, I find this declaration odd to say the least: the vast majority of successful amateur aquarists don’t even use an UV sterilizer so they clearly can’t be that important. And the mention of a mechanical filter I also find odd, because I only use them as a prefilter to my biological filter, which I would surely rank as the most important aquarium device anyone can have.

If the opening statement was meant to boost sterilizer sales then I am afraid that in my case, at least, it has failed, since the contents of the sterilizer chapters have convinced me not to buy one. The discussions on “dwell time” and “zap dosage” clearly show the futility of using a small UV sterilizer in a moderate to large sized aquarium system. For example, a 25-watt UV sterilizer (itself costing about $250) can only be useful on a 180-liter or smaller aquarium, and then only if the flow rate through the sterilizer were carefully regulated. My 500-liter show tank would require 64 watts of UV to be effectively sterilized at a cost of nearly $1000 (including pump, flow monitor, and plumbing). Since I have never seen any of the diseases that UV sterilization is supposed to prevent, this expense hardly seems justifiable. However, to anyone deciding on using an UV sterilizer, I would say that these chapters are required reading, as they convincingly demonstrate that careful matching of the tank capacity, sterilizer wattage, and pump flow rate is required for satisfactory results. You simply can’t stick any old sterilizer to the output hose of your canister filter and expect effective sterilization.

The following chapters discuss the design and operation of protein skimmers, and should required reading for anyone wanting to design such a device.

There is also a chapter that discusses heating and cooling of aquaria. Heat loss from a model aquarium is examined and examples are given as to how to determine the heater wattage needed for a given tank. This information is potentially very useful, but unfortunately it doesn’t adequately address all the complications that arise from extraneous heat coming in from the lights or water pumps, or the complex configurations of multi-tank systems, or of open-air tanks that suffer from evaporative heat loss. So in the real world, you would still probably have to rely on trial and error when sizing a heater for a multi-tank system.

Two points do however become quite clear while reading the chapter on aquarium heating: acrylic tanks require much smaller heaters than do glass tanks, and
“watts per gallon” heater sizing rules are useless. Heat is lost from the tank proportionally to its surface area, not its volume. So even though beginner books may advocate buying a heater big enough to supply “5 watts of heat per gallon”, and even though this may be fine for a 10 gallon tank (50 watts of heat), it results in serious overkill in 100 gallon tank, where 500 watts of heat will cook your fish.

The chapter discussing water pumps is also informative, but is hampered because it discusses pump performance in terms of output pressure. Unfortunately pump output pressures are almost never given for hobbyist pumps. Instead, pumps are rated by flow rate…either at a variety of “heads” (if you are lucky) or as a single value. The pump “head” is the height the water is lifted by the pump, but many aquarium pumps are designed strictly for “flat flow”, or zero head. Such “circulation pumps” are only given a passing mention.

For those pumps that are rated for pump flow at various heads, it is easy to determine the pump pressure, but the relevant equation is not provided in the book. So here it is…let

P = pump output pressure in Pascals.

z = be the height in meters where flow rate drops to 0 (the maximum head).

d = density of water (which is 1000kg/m3 for fresh water)

g = gravitational acceleration (which is 9.8 m/s2 near the Earth’s surface)

then P = d g z

= (9800 kg m-2 s-2) z

To convert pressure in Pascals to PSI, multiply the pressure in Pascals by 0.000145 PSI/Pa. You can also make a quick estimation of the pressure by remembering that every meter of head requires the addition of about 1/10 of an atmosphere of pressure, where 1 atmosphere is about 1000 Pascals or 14 PSI. For example, my pond pump is reported to pump to a height of 8 meters, so it must deliver about 11 PSI.

Once the pressure of the pump is determined along with the diameter of the pump outlet, the flow rate at any head is easily determined from the formulae presented in this book. But much more importantly, the book also presents the information required to allow you to design a real filter system, taking into account the losses of flow due to friction in your hoses and connectors.

This information is to my mind the most universally applicable information in the book, as the most complicated piece of “aquatic engineering”” that the advanced hobbyist is likely to attempt is a single-pump, multi-tank fish room. Everything you need to know about sizing and designing such a room is included in this book, and I don’t think anyone should attempt it without first having a thorough read. ?

Some Considerations Before Designing Your Own Fluidized Bed Filter

As aquarists, we are all aware of the need for biological filtration and most of us also understand the principles involved as well. Biological filtration is the term used for the consumption of fish waste by biological means, but as aquarists we usually limit the term biological filtration to the oxidization of ammonia by aerobic bacteria to produce nitrate. The process of oxidization of ammonia is called nitrification. Ammonia is excreted by fish through their gills and in their urine, but it is also produced by heterotrophic (food-eating) bacteria that consume solid fish wastes. Nitrification is a two step process in which ammonia and ammonium (NH3 and NH4+, respectively) are oxidized by bacteria to produce the nitrite ion (N02-), then the nitrite is oxidized by other bacteria to produce nitrate (N03-), thus completing the process of nitrification. It is often stated that Nitrobacter and Nitrosomonas bacteria accomplish these tasks, but it has recently discovered that this is not the case, at least in freshwater aquaria. We don’t know yet actually what bacteria are involved in nitrification, but they are not Nitrobacter and Nitrosomonas. Regardless of which bacteria are involved, it is however known that they require a solid surface on which to grow.

The traditional nitrification filter for home aquaria is the under gravel filter (UGF). These are relatively simple devices in which water is drawn from below a false bottom under a layer of gravel. Water thus flows down through the gravel so nitrification takes place on the surfaces of the gravel grains. However, UGF is not entirely satisfactory in large aquaria or aquaria with high fish densities because the fishes’ solid wastes are trapped in the gravel. This results in the gravel clogging up. Under gravel filters are also generally considered to be detrimental to plants, although a lot of debate still goes on as to how much harm to plants UGF actually does.

A vast improvement over UGF is wet/dry filtration (WDF). In WDF, mechanically filtered tank water is allowed to trickle over emersed (“dry”) filter media. Since the filter media are exposed to the air, the growth of bacteria in these filters is never oxygen-limited. These filters therefore have tremendous biological capacity. Wet/dry filters also serve to oxygenate the water, unlike UGF, which depletes water of oxygen.

The primary difficulty with WDF is that the filters are physically large. They require a volume that is significant fraction of the tank volume in order to handle the ammonia produced. A much more space efficient design is a fluidized bed filter (FBF).

Fluidized bed filters consist of a vertical contact column in which water is pumped upward. Sand placed in the contact column is thus suspended by the upward flush of water. The trick is to supply a sufficiently rapid water flow to suspend the sand, but not so rapid that the sand is blown out of the top of the filter all together. The sand supplies a surface on which the desired bacteria may grow. A FBF is also self-cleaning since the sand is in constant motion and so no detritus can settle on it. This is of limited benefit however, since the detritus is simply flushed out of the filter and ends up back in the tank, where you don’t want it either. Well, at least it doesn’t clog the filter up. But the greatest advantage of FBF is the great surface area provided by the sand. Since sand has a small particle size the surface area available per unit volume of sand is very large. Spherical sand with a grain diameter of one mm (i.e. medium coarse sand) has a surface area of 3141 square meters for every cubic meter of sand. So what? Well it means a couple of handfuls of sand have as much surface area as a typical wet/dry filter.

FBF systems are not without their own problems however. A wet/dry filter makes better use of its relatively limited surface area by growing a thicker coating of bacteria than is possible with FBF. The mutual abrasion between moving sand particles in FBF limits the depth to which bacteria may grow on the sand. FBF also share a disadvantage of UGF in that they deplete the water of oxygen. Fluidized bed filters are also potentially dangerous. When the water pump stops because of power or mechanical failure the sand will settle to the bottom. The packed sand can quickly become anaerobic, resulting in the death of the desired aerobic bacteria. Anaerobic sand may release highly toxic hydrogen sulfide, sometimes within hours of pump failure. However, you can prevent the filter from dying in a power outage if you have a backup generator, or even a battery-operated air pump with which to feed air to the bottom of the sand filter. This will keep the sand moving around and oxygenated enough to prevent problems from developing, at least temporarily.

Fluidized bed filters are now commercially available in sizes appropriate to home aquaria. However, if you are like me, after seeing the commercial FBF units pictured, you would have thought to yourself, “They look pretty simple: why not build one myself?”. Well, the good news is that you can, but the little devils are trickier than you might think. Here are a few things to consider when building your own FBF.

There are many designs that can be used in the construction of an FBF. The simplest design is a simple vertical column. Sand in these cylinders flows in a sort of circulatory motion, up in the center and down along the walls. This circulation quickly breaks down if the water flow gets too great, as sand will then simply blow out the top of the cylinder. Therefore, simple cylinders require careful flow adjustments to provide a suspended sand bed. They are therefore the most difficult to tune, even though they are simplest to build. Another design, which is fairly easy to construct, is the V-shaped trough filter (shown below). A cone design can also be used. And another of the many possible designs is a wide topped cylinder, which is similar in shape to those short ice cream cones with the flat bottoms.

All successful FBF must provide a sufficiently high upward water velocity to fluidize the sand, as well as a region of sufficiently low upward water velocity for the sand to settle back into the filter. A useful rule of thumb is that the water velocity required to fluidize quartz sand in sea water, measured in meters per minute, is equal to the sand grain diameter measured in millimeters. Therefore a filter with sand of grain size one mm requires a water velocity of at least one meter per minute to suspend the particles.

The ability of water to fluidize sand is however also dependent on the viscosity and density of the water, so the rule of thumb has to be modified for water of different temperatures or salinities. The rule is valid for seawater at a temperature of 20C, but the temperature correction is small and can be ignored within the narrow temperature range permissible for a tropical aquarium. The differences in density and viscosity between seawater and fresh water are however more important than those created by temperature changes. The velocities required for fresh water are 13% larger than those needed for seawater. A fresh water filter with a sand grain diameter of one mm therefore requires 1.13 m/min.

So now lets design a FBF for a large tank. This will illustrate some of the design problems that have to be considered when constructing your own filter.

First of all, a design type must be chosen. Let’s reject the simple cylinder design since they are too prone to losing their sand out of the top. The conical and topped-cylinder designs are not easily constructed from commercially available plumbing fixtures, so they are rejected as possible designs also. That leaves the V shaped trough design. They are not nearly as compact as cylinder designs, but (being flat-sided) can be easily constructed with plate glass, acrylic, or even suitably water-proofed plywood.

Lets put the filter behind the aquarium and have it sitting level with the tank bottom. We don’t want to have to drill any holes in the aquarium, so the influent water has to be supplied by a siphon. For simplicity and reliability, a simple overflow pipe will be used to return the water to the aquarium. The height of the filter must therefore be about the same as, put slightly higher than, the aquarium itself to allow the overflow pipe to be positioned over the aquarium top.

Assume that the filter is to be serviced by a water pump with a flow rate of 2000 liters per hour, which is 0.033 cubic meters per minute. The filter is be filled with 1 mm diameter quartz sand. Recall that the water velocity in the contact column must therefore be at least 1.13 meters per minute to suspend the sand in a fresh water, tropical aquarium. So to suspend the sand the trough must have a cross sectional area near the bottom of less than 0.04 m2 (= 0.033 m3/min * 1.13 m/min). Conversely, the top of the trough must have a cross-sectional area considerably larger than 0.04 m2. So for this filter, we could use a V with an open top of 20cm wide by 40cm long (giving an area 0. 08 m2). Pumping 2000 liter per hour, this filter can comfortably service a 500 to 1000 liter aquarium.

We could not fill this filter with sand to any more than the height where the width of the V is 10cm (where the area is 0.04 m2), because filling the V with any more sand than that will prevent the sand from fluidizing. How much sand must we put in? As a general rule, the fluidized sand will have about twice the volume as does the same sand when it has settled down to the bottom, so you should about half fill the filter with sand (or about to where the V is 10cm wide in this case).

One difficult part of building your fluidized bed filter is finding suitable filter sand. Graded filter sand is commercially available for water treatment plants and other purposes, and sandblasting sand is also graded for size and is suitable. However such commercial sources will usually sell their sand in 100-pound bags, or by the cubic yard or cubic meter. Graded filter sand is also expensive, usually costing about $200 per cubic meter, although the cost varies greatly with the uniformity of the sand particles. It is however not necessary to buy graded filter sand as you can grade your own. Quartz river sand is best since it is rounded, but beach sand can also be used if it is relatively coarse and is free of shells. A bug screen in a wooden frame can easily be used to separate sand particles larger than 1 mm from your sand. These large grains must be removed since they won’t fluidize properly. Removing sand particles that are too small is however more difficult, since suitable screens are not readily available. Fortunately, it is only necessary to remove those particles that are too small to remain in the filter under normal operations. Therefore you can grade the sand adequately by simply running the filter and discarding those sand particles that are pumped out of it. A useful method is to fill a bare aquarium with water out on the lawn. The FBF is then attached so that water flows up through it as normal, but then simply flows out the overflow pipe onto the grass, rather than going back into the tank where the fine sand can damage the pump. This process drains the aquarium rapidly, so keep the aquarium full with a continuously running garden hose. Keep it up until the water runs clear. Not only does this water your grass, it adds sand to your lawn (usually a good thing)

Once constructed, the filter must be run continuously, even if the filter is unhooked from the tank. When not in use, a hose connecting the inlet and outlet can be put in place, thus allowing the filter to cycle water through itself. Or, as mentioned, an airstone buried in the sand will keep it alive for the duration of most emergencies

I have deliberately cut short of giving detailed plans for the construction of a FBF. I did this for a few reasons: First of all, you might build a filter to be exact specifications and then you’ll blame me if you are unhappy with it. A selfish reason to be sure, but I also think its a lot more fun to design your own filter using parts you find yourself. Remember that when designing your own filter, the most important thing to determine is the water velocity through the contact column and the water velocity required to fluidize sand of your grain size.


Levich, V.G., 1962, Physiochemical Hydrodynamics, Prentice-Hall Inc.

Spotte, S.H., 1979, Sea Water Aquariums, Wiley-Interscience

Wheaton, F.W., 1977, Aquacultural Engineering, Wiley-Interscience?

Waterfalls (For Ponds – ed. note)

When I first started my pond two years ago, the only aeration/moving water was from a small fountain and sprinkler head with an attached foam filter sitting in the middle. Although this was adequate to keep the fish alive and the pond from turning into a scummy slough, I would have preferred something that looked a little more natural. Next year I had a waterfall.

One of the more common ways of getting a waterfall is to buy one ready made, but if you’ve ever priced some of the molded fiberglass waterfalls, you’ll know most don’t come cheap. The ones we can usually afford are rather small as well. Sometimes they are no more than miniature water slides, and must be positioned and landscaped so they are sitting a few inches above the surface in order for water to “fall” into the pond. Some people have the patience and talent for such landscaping, but there are other ways to get a waterfall.

I chose a “home-made” approach. All you are doing is making a few small pools, setting them up so they slightly overlap, and pumping pond water to the top one. Each pool has a notched edge where the water drains out into the next, and then back into the pond. The bottom of the basin is made of plywood, and the sides are made of landscaping timbers. It took me (and another person) about 4 to 6 hours of construction and the whole thing was ready to put up.

The first thing you must do is decide how big and what shape to make the basins. Mine were a hexagonal shape, and had an outside diameter of about 32 inches. This tells you how much material you need. Three-quarter inch plywood, which costs about $35 per 4X8 sheet is fine. Depending on the size of basin you choose, you can get away with 1/2″. Landscaping timbers cost around $3, and are 8 feet long. Both can be found at building supply stores. Although a miter saw and a circular saw are the most efficient tools for making the necessary cuts, all that is needed is a hand saw and a bevel to achieve the same results. You don’t necessarily need to waterproof or treat the wood, since it will have a layer of plastic over it, but the pools will last longer if you do (4 to 5 years as opposed to 10+).

To find the angle at which to cut the timbers, take 360 degrees and divide it by the number of sides your pools will have. Before making any cuts, decide how long to make the sides. One thing I did was to cut out a paper hexagon the same size as the basins beforehand, both to see how big they would really be, and to make sure I had room to fit them where I wanted them.

Once you’ve decided on a length, (say, 20 inches), and a shape (say a hexagon), cut the plywood as shown. Next, cut the timbers for the sides as shown in Figure 2 (use the angles for the shape you’ve chosen here).

You must also choose how deep to make the pools. I would recommend (if you’re using timbers) two layers, or 6 inches deep. One layer will make the basins more like plates, three or more layers deep and they tend to get too big and heavy to handle.

Since the timbers have two flat sides, they are easily attached to the plywood. It’s best if you attach the bottom row from the bottom with 1.5 to 2 inch #8 or #10 wood screws. To attach the second row, I drilled a small pilot hole through the top timber and attached them to the bottom row with three-inch screws.

When you have everything together, cut a drain in the top of one side in each basin. You can do this before or after assembly, since these basins (when properly made) are very solid. The notch shape affects how the water falls. A downward sloping, fan-shaped notch will spread the water out in a wide sheet, while a downward sloping pointed notch will drain the water like milk flowing from a carton. An upward- sloped or even level notch of any shape will only let water slop down the edge of the basin like an over-flowed cup of water, which will rot the wood. Make sure the water flows over an undercut edge such as the rim of the plastic or even a piece of tin under the metal, or again it will slop down the front of the wood rather than flowing into the next pool. The size of the notch depends on how fast the water is being pumped in, but starting with a smaller notch and increasing it’s size, if necessary, through trial and error worked for me.

To waterproof the inside, all that is necessary is one layer of 6 mm poly plastic stapled down or secured under a small strip of wood around the rim of the basin. Six mm refers to the weight of the plastic, and you’re safer with thicker plastic. You’ll have to replace it each year, but a small role of this plastic (several hundred square feet) only costs about five or six dollars. Pond liner can be used too, but it is expensive, plus you have to cut it in a shape and size to fit the basins. Even if you can figure out the dimensions, more than likely you will have a lot of big folds and a tough time getting it in.

When the basins are done, they must be set up so they drain into each other and the pond. This can be done many ways, but if they’re not flat on the ground (a hill), make sure that they’re sitting on something solid. Full of water, most will weigh over 100 pounds. Mine were set on cinder blocks at varying heights. Because you have separate units, the waterfall can be set up in many different ways and patterns. That’s up to you. Something you should do is make sure they are level so they drain where you want them to, not all over the ground. You must be careful doing this as I have first-hand experience in the mess you can make when the pools are not “level enough”.

Make sure your pump can handle getting the water from the pond up to the top of the falls without wearing it out. Once there, it can go through a filter, or directly into the basin. I’ve got my filter at the top of the waterfall behind some potted plants and a tree branch, where it isn’t very obvious.

Depending on how far the water falls from basin to basin, a few drops might splash out onto the ground. Such a small amount of water will evaporate before making any mud, and is usually replaced by rain. To make the whole thing look better, you can set potted plants around the rim of the basins. The plants are constantly being watered by the splashing water, and give the waterfall a more natural look, particularly if they have flowers.

The best asset of this kind of waterfall is that it can be changed around very quickly and easily. The basins can be set far apart to be like a small stream, or almost on top of each other for a vertical drop. Their numbers and size are also flexible. The basin’s size allows them to be movable, but they are solid enough not to need additional landscaping or heavy rocks to keep them in place. ?