A few people have asked me what I use as a substrate in my plant tanks, and what is the best substrate for healthy plant growth. After trying a few different options and seeing how other people grow their plants I’ve come to the conclusion that just about anything will do. It comes down to taste, the type of fish kept in the tank, availability, cost, and the amount of maintenance required.
In an aquarium there really isn’t much water circulation through the substrate unless we use undergravel filters or substrate heating. It is this circulation that keeps the soil aerated. Without it, the soil could run out of oxygen (become anaerobic) and turn black. Organic debris decomposing in this environment would ferment and release hydrogen sulfide (that noxious rotten egg smell). This is just what we don’t want happening in our tanks. And so we can not use just any natural substrate for our aquarium plants. In nature aquatic plants grow in gravel, sand, clay, and a mixture of roots and decomposing matter. But let’s assume we don’t have any substrate circulation in our aquaria. In such a situation, very fine sand and a mixture of roots and decomposing matter will definitely go anaerobic. Clay isn’t a good choice either because any water movement near the bottom would stir it up or cloud the water, although clay can be used if it is topped with a layer of sand or gravel. This leaves us with different grades of sand and gravel.
Now that we have narrowed our choices, what should one use? First, one must make sure that the material won’t react with the water. It must not dissolve or leach anything. A few drops of vinegar or lemon juice will indicate if the material contains a carbonate by producing bubbles. Carbonates should not be used because they dissolve and make the water hard and alkaline (fine for East African and Central American cichlids, but not the best for plants). Also, the material should not have sharp edges as these are abrasive to fish, plant roots, and the aquarist’s hands.
Secondly, one has to decide what grade of sand or gravel to use. Horst and Kipper (1986) recommend using gravel 2 mm to 3 mm in diameter. Scheurmann (1985) recommends using sand 1 mm to 2 mm in diameter. From personal experience, I’ve had success with both sand (mixed grade ranging from less than 1 mm to 2 mm diameter) and red flint of 2 mm to 4 mm in diameter. Some club members are very successful using gravel with grain sizes of 5 mm and larger. I find that roots grow finer and more extensively when sand is used. One thing to avoid for sure is sand with grains consistently in the 1 mm and less range. Such sand tends to compact and prevents roots from growing; it also becomes anaerobic very easily.
For aquarium purposes, sand and gravel differ in many ways beside their relative grain size. Here I will define sand as having grain sizes of 2 mm and smaller and gravel as having grains 2 mm and larger. When added to water the two behave differently. Sand packs together and stays soft if the majority of the grains are on the larger side, or packs solidly if the majority of the grains are on the smaller side. Gravel, on the other hand, stays loose with gaps between the grains. The larger the grain size, the larger the gap size.
When it comes to setting up, sand demands some special attention and creativity. It holds more dust when dry, so it takes longer to wash. A standard undergravel filter won’t work with sand, and perish the thought of sand being sucked by a powerhead! I have read on the Internet of an aquarist who wrapped fabric on the undergravel filter plates and used it successfully, though reverse flow may be out of the question. As for powerheads, just keep the intakes a few centimeters higher than the sand surface. A stone under the intake will keep sand from being sucked in. If you hear grinding noises from the powerhead, dismantle it and clean it thoroughly. Sand in powerheads is not necessarily fatal, but you don’t want it going on for any length of time.
Cleaning the aquarium is easier with sand. Because sand packs fairly tightly, debris stays on top of it. Therefore it is not necessary to “vacuum” sand as one would do with gravel. A quick pass with a siphon hose is enough to suck up the debris with a little sand. When finished, just wash whatever sand was sucked out and put it back in the aquarium.
Setting up with gravel is straight forward, and gravel is less likely to get sucked into filter intakes. With the larger sizes of gravel, vacuuming becomes important. All those gaps between the grains will fill up with debris and the substrate could become anaerobic. Plants with heavy root growth will help here, but one must make sure that open areas are clean of debris. Don’t vacuum too much around plants as the debris provides nutrition to them.
Sand is available at aquarium shops and as a construction material from hardware and landscaping stores. The selection is not great. The most common is a smooth quartz sand, beige in color, with wildly varying grain sizes (from dust to just over 2 mm). I have also seen for sale a white carbonate sand, which is not suitable for our purposes here. The effect of the lighter color of sand on the fishes’ colors can be offset by heavy planting and a dark background. Sand is very cheap at hardware and landscaping stores. A 25 kg bag of “play sand” (enough for two 15-gallon tanks) costs less than $3.00. Unfortunately you never know what variation in grain sizes you will end up with (lately it has been too fine). Sandblasting sand is more expensive, but the grain size should be more consistent.
Gravel is much more diverse in its availability. It comes in all colors of the rainbow, black, white, and anything in between. Some types of gravel are smooth, others look like broken glass. I’ve had some black gravel that was actually clear gravel with a black coating that would wear off slowly. Look for something that has a natural look to it and is not too coarse. Darker gravel makes fish look darker and more colorful. The flip side of gravel is its cost. Most of it is only available in smaller quantities at aquarium stores, so costs are higher. Natural gravel is available at construction supply stores at more reasonable prices, but you could end up with a mixture of rock types, some being carbonates.
When it comes to fish, any fish that burrows or eats from the bottom will love sand. Eels, loaches, and whip-tailed or banjo catfishes will bury themselves in sand. Corydoras will dig in it most of the day and their barbels will stay in great shape. It is advisable to have fish that will sift the sand because the top layer may develop a film of algae and become encrusted; constant sifting will keep the sand loose. Other fish don’t seem to care if the bottom is sand or gravel.
Bottom-dwelling fish that come from areas with strong currents prefer gravel or even rocks. Burrowing fish won’t do as well in gravel, especially if it is coarse or has sharp edges. I’ve seen Corydoras without lips, much less barbels, in tanks with round-grained gravel but with a coarse texture.
For the record, I have used red flint in my tanks and had an underwater jungle growing in my 60-liter tank. I liked the color and texture of the gravel, but not the price. I used sand when I set up my large tank and now I will not go back to gravel. All my tanks have sand in them now. I like the look and price of sand, and most importantly my plants and fish are thriving.
In conclusion, the choice of substrate is really yours to make. This article hopefully provided you with some useful information. I will discuss substrate additives and water circulation in future articles.
Horst, Kaspar, & Kipper, Horst E. (1986). The Optimum Aquarium. Bielefeld, Germany: AD aquadocumenta Verlag GmbH.
Scheurmann, Ines. (1985). The New Aquarium Handbook. Woodbury, NY: Barron’s Educational Series Inc.?