Modifying Water Chemistry

by Grant Gussie, CAS

originally published in The Calquarium Volume 40, Number 10, July & August 1998

The article "Water Chemistry, For Dummies" was published in the May issue of The Calquarium. While it was (perhaps too) long on theoretical information, it was short on practical application. So this month, lets deal with practicalities. So now that you know what pH, alkalinity, and hardness all mean, how do you go about changing them?

As it turns out, increasing all three is very easy. Therefore if you live in a soft water region and want to keep hard water animals (like African rift lake cichlids) you are in luck.

Baking soda (NaHCO3) when dissolved in water will add bicarbonate ions and thus raise the alkalinity of the water as it raises the pH up to about 8.2. Dissolving 1 mL of baking soda in 10 liters of water (or 1 teaspoon in 10 gallons) will raise the waterís alkalinity by about 110 mg/L CaCO3 (it has no effect on hardness).

Hardness is easily increased as well. Epsom salt (hydrated MgSO4) is inexpensive, readily available, and 1 mL of it in 10 liters of water (or 1 teaspoon in 10 gallons) will increase permanent hardness by about 70 mg/L CaCO3. However you may wish to increase the calcium (Ca++) contribution to hardness too. An aquarium substrate of calcitic gravel will increase the hardness of acidic water, but because calcium carbonate is so insoluble in alkaline water, it will have little effect in hardening water with a pH much above 7.5. By all means use a calcitic substrate in rift lake or marine tanks ─ it is very good insurance (preventing the pH from dropping below 7.5) but it will have no other appreciable affect on an alkaline aquarium. Raising calcium levels in alkaline fresh water is however not that difficult. Just a little marine salt mix will raise calcium to a level suitable for any fresh water tank without also raising the salt concentration to unhealthy levels. Calcium chloride also does the trick safely and inexpensively, but calcium chloride is not readily available in local stores. But you can buy it from chemical supply stores.

These methods are however unsuitable for salt-water reef aquarists. They want to keep stony corals and giant clams alive, but these animals will quickly deplete water of calcium if it is not renewed. Reef aquarists consequently want a source of calcium supplementation that is soluble at an alkaline pH, but does not also add other ions that will change the chemical composition of the water over time.

A calcium compound that is soluble at a higher pH is calcium hydroxide, and so it is sold to reef aquarists for this purpose. However the price pet stores ask for this usually very inexpensive (but not readily available) chemical is prohibitive. So is the fact that calcium hydroxide is one of the most caustic chemicals available and potentially very dangerous to use. Many reef aquarists have now-a-days stopped using calcium hydroxide as a calcium supplement and are instead using a CO2 reaction chamber filled with calcitic gravel. The CO2 injected into these devices acidifies the water, which then dissolves the calcium carbonate (CaCO3) to raise calcium levels and alkalinity while returning the pH back to its original value of about 8.2. This provides a much safer way to raise calcium levels than adding calcium hydroxide.

So to raise pH, hardness, and alkalinity in fresh water you are well advised to use equal parts of baking soda, Epsom salts, and either calcium chloride (if you can find some) or a commercial marine salt mix. Add enough of this mixture to raise the pH to the desired level. One mL salt per 5 liters of water (or 1 teaspoon per 5 gallons) is usually sufficient to raise the pH to 8.2. To raise the hardness and alkalinity of salt water, it is best to go to the trouble and expense of acquiring a CaCO3/CO2 reactor. And for both, use a calcitic substrate to prevent dangerous drops in pH.

It is however much more common (at least in the Prairies) to want to lower pH, alkalinity, and hardness, rather than raise them. This is because water sources east of the Great Divide and west of the Precambrian Shield are all in close contact with calcitic rocks, and therefore are as hard and alkaline as any you are likely to see.

Consequently, hard water is an annoying fact of life for many people, not just aquarists. Hard water reduces the cleaning effectiveness of soaps and also leads to the build up limestone scale in showers, kettles, and steam irons. Many companies will capitalize on this and try to sell you devices to reduce water hardness. Some commercials for these products will even imply that drinking hard water is bad for you. Donkey detritus. Hard water is perfectly good to drink, and in fact, its better for you than drinking distilled water. For example, drinking a liter of Calgaryís water will supply 5% to 10% of your daily calcium needs. But unfortunately most of the fish and plant species we keep are from the soft acidic water found in the rainforests of Southeast Asia, West Africa, and South America, and as such, our hard prairie water is not ideal for them. I would however recommend that you do not try to do anything about this fact unless you are an advanced aquarist with specific goals in mind. Commonly kept beginnersí fish will adapt to our water just fine.

However, advanced aquarists may wish to breed those few species of fish or plants that really do need soft, acidic water; or they may undertake other projects, such as producing the highest possible growth rate of their plants. These goals are completely legitimate and if you are ready to take on the challenge, feel free to modify your water chemistry.

I would like to strongly suggest, however, that you not do lower pH by putting something into the water, with the exceptions of peat extract and CO2. And if you use CO2, be very careful and be prepared for fish losses if your CO2 injection breaks down. The reason for this is simple; by lowering pH without simultaneously buffering the water to the lower value, you are depressing pH below its natural level. This creates an inherently unstable pH. When the pH (inevitably) returns to its natural level, your fish will be stressed by the changes in water chemistry.

So if you want to lower pH, be prepared to lower alkalinity as well. But be forewarned that this is not without its own potential dangers. The lower waterís alkalinity is, the less stable its pH becomes. This is because fish produce acidic wastes that can dangerously lower pH in low-alkalinity tanks if these wastes are allowed to build up. You must monitor pH more closely and water changes become extremely important if you lower your tankís alkalinity. If you find that the pH of your water drops continuously, you must reduce the fish load, do more or larger water changes, clean the filters more often, and consider raising the alkalinity to a somewhat higher level.

Lowering tank alkalinity is not a simple proposition. The easiest and (in the long term) most cost effective way to do it is to use naturally soft water to begin with. Then you can add either enough hard water or baking soda mix to raise the water to the desired alkalinity level. Sources of suitably soft water are ion exchange, reverse osmosis, distillation, rainwater, and melted snow. Unfortunately, rain and snow really arenít viable water sources in a city as large and dry as Calgary. There is just too much dust and grime in the air and not enough regular precipitation to reliably wash it all out. And distilled water is likely to be too expensive to be purchased in the quantity an aquarist would need. Reverse osmosis is a better option. The problems with reverse osmosis units are however that the equipment is fairly expensive (several hundred dollars at least), they consume at least five times more raw water than they produce as pure water, and they never produce as much pure water as they are rated for. But if you can afford them, they make nice pure water reliably and simply.

Alternatively, a cation exchange process can be used to soften water. This is the kind of process that household water softening machines and those ion-exchange pillows use. These may or may not also lower alkalinity through an anion exchange process. In cation exchange processes calcium cations are replaced with sodium cations, and in anion exchange processes bicarbonate anions are replaced with chloride anions. This makes the water softer, but also saltier. Typical household water softeners will almost double the concentration of dissolved ions in the water. Softening water in this manner is therefore of little benefit to soft water fishes, as they will suffer as much from excessive salt as they do from excessive hardness. One should also note that those units that only soften water (cation exchange) and do not simultaneously lower alkalinity (anion exchange) will be of absolutely no help in lowering the waterís pH, and so they are of no benefit whatsoever to fish from soft acidic waters. If the unit does not specify that it performs both cation and anion exchanges, it is a waste of money.

Another type of ion exchange process is used in commercial water-purifying units. These units replace HCO3- anions with OH- anions and Ca++ cations with H3O+ cations. Because the OH- and H3O+ will then combine to form H2O, these resins actually remove ions from the water, rather than simply replacing one ion with another, as do water softeners. The problem with these resins is however that they are far too expensive to use without recharging them, and they require two very caustic chemicals to recharge them; hydrochloric acid and caustic soda. The anion and cation exchange resins must also be separated before they can be recharged. Many manufacturers deliberately mix the two resins in order to prevent you from recharging them (they make more money that way). Fortunately you can often thwart the manufacturerís intent as the two resins can be re-separated by floating the beads in salt brine of the proper density. You just keep adding salt to the brine until the anion beads start to float and the denser cation beads stay on the bottom. Once separated, the beads can be recharged by soaking them in the proper caustic chemical solution. It is debatable however whether or not doing this is worth the bother when a reverse osmosis unit can be purchased for a few hundred dollars. The expense and danger of recharging the beads seem to make this an impractical process for most aquarists.

Once you have water of suitably low alkalinity, you can lower the pH to a suitable level. But a problem arises, you not only want to lower alkalinity, you want to increase acidity to make the water well-buffered and stable at its acidic pH as well. I would therefore not recommend adding chemical acids (such as hydrochloric acid) that lower pH without increasing acidity. These chemicals will only create a rapidly changing and unstable pH.

Unfortunately raising acidity is difficult. Commercial products intended for this purpose can be found on pet store shelves, but almost all of them use sodium biphosphate as a buffering agent. Sodium biphosphate does indeed buffer the water to a nice, mildly acidic pH of 6.5 and would therefore seem to fill the bill nicely, but the problem with sodium biphosphate is that the phosphate serves as an inorganic fertilizer for algae growth. I would therefore not recommend using phosphate-based acidifiers. You might however be able to find a commercial non-phosphate acid buffer, which should be fine. Seachem® and Kent Marine® reportedly make such things, but I canít vouch for how well they work. But reports on the Internet seem to give these products good reviews. Also, I would recommend filtering the water through pure sphagnum peat moss and possibly to also use CO2 injection.

Peat moss will release organic tannins, and thus increase acidity while it lowers pH. It is consequently a relatively safe method. Some people object to the brownish color peat imparts to the water, but for most (if not all) soft-water fishes, such tea-stained water is perfectly natural and healthy for them.

CO2 injection is potentially a more dangerous method of pH reduction than peat filtration because it lowers pH without raising acidity. Therefore, if the CO2 injection ever stops, the pH will increase as the excess CO2 is lost to the atmosphere. But CO2 injection is known to greatly stimulate plant growth in tanks with abundant light, and it is for this reason that aquarists are willing to take the (slight) risk involved with CO2. If you use CO2 injection, accept the fact that you are lowering pH without increasing acidity and that there are risks involved with doing this. Be careful about water changes as well, since the replacement water will have a higher pH than the CO2-laden tank water unless CO2 is injected into it too.

The concentration of CO2 and the concentration of HCO3- and CO3-- are all closely linked and dependent on pH. And because HCO3- and CO3--account for almost all of the alkalinity of unpolluted water, there is a close link between the pH, alkalinity, and CO2 levels as well. It is therefore possible to construct a table in which the measured alkalinity and pH will allow you to determine the concentration of CO2. The CO2 concentration of the water is therefore easily determined if you know the pH and the alkalinity of the water, both of which can be measured by relatively inexpensive test kits (see figure 1).

The book The Optimum Aquarium by Horst and Kipper describe a CO2 injection system designed to optimize plant growth. An alkalinity of the water of 50 mg/L CaCO3 (about one third that of Calgary tap water) is maintained in their system, and the pH is lowered to 7.0 through CO2 injection. So if you want to recreate the water used in Horst and Kipperís fabulous show tanks, just dilute Calgary tap water with pure water at a ratio of one to three, then invest in a CO2 controller system and set it to pH 7. Simple enough, but the CO2 system will set you back about $2000. Fortunately, a little trial and error with the yeast CO2 method (c.f. Wlad Franco-Valiasí article in the February 1998 of The Calquarium) will do the job much more cheaply. For optimum plant growth, you want a CO2 concentration that is in the range of 10 ppm to 20 ppm. This is considerably higher than the 0.6 ppm that results from aerating the water conventionally (i.e. with air). It is important to limit the CO2 however, as concentrations of CO2 higher than 35 ppm are dangerous to fish. If your water has an alkalinity greater than 200 mg/L CaCO3 then lowering itís pH down to 7.0 would require more than 35 ppm CO2 and so doing this is not recommended. In the case of such a high alkalinity (which can be achieved by Calgary tap water in winter) dilution with pure water is mandatory. And always remember, with CO2 injection you are depressing pH below its natural level and so you are running the risk of fish losses if the injection system ever stops.?


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