Precautions for the Use of Soil Substrates

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The following information is a summary of precautions that I have prepared for the use of soil and clay substrates. It was originally prepared as advice for a subscriber to the Internet Aquatic Plant Digest mailing list. He was experiencing troubles with fish in a newly set up aquarium. It turned out that the problem wasn't related to his substrate however I felt the advice was suitable. It's primary perspective is from the standpoint of the safety of the fish but I have expanded on it to cover a few other factors. 

1. Know a little about your soil. You can refer to the links on my web page, http://home.infinet.net/teban/ for more information about soil science. Soil taken from a well drained, forested or meadow location is a good choice. Go for a hilly spot. This helps to ensure that the soil is not too high in soluble nutrients.
2. Test a sample of the soil for alkalinity. Soil that is derived from limestone is often low in iron. Sometimes a particular mineral soil might contain some soluble minerals, especially alkaline ones, which we need to think about when collecting soil. I suggest putting a soil sample in a container of water and checking for pH reactions. That will help detect carbonate salts. Sometimes an alkaline soil in an arid region like a desert or an alkaline bog also accumulates other soluble metal salts like copper, mercury and lead so that's why the pH test gives another clue.
3. Submerge the soil for a while and then change the water. You can do this before you install it in the tank. This also helps to stabilize organic soil components, which will undergo biological decomposition most rapidly for 3-4 weeks. I don't install fish into a new soil tank right away. I usually add an expendable test fish first such as a Platy (they breed like mad, oops! off topic ;-) a few days after the tank is planted. I also perform a couple of 90% water changes before putting the fish in for a few days. That may be over cautious but I figure that the water is cheap. Never be in a rush, eh?
4. Use some topsoil that contains humus or peat. A soil with some humic material will sorb dissolved metal ions which otherwise might cause micro nutrient toxicity. That is one of the reasons that I use peat (Canadian Sphagnum Peat Moss), mixed in a thin layer in my substrates. Peat is not quite as stable biologically as soil humus but its close especially after a few months under water. I suggest using about 1 part by volume of fluffy peat to 3 parts of the soil mixture. This should be approximately 2-5 % by weight of peat in soil.
5. Don't bury organic material deep. Oxygen only penetrates a shallow distance into the substrate and if you have a deep layer of peat or garden soil, the action of bacteria on this organic material (however well decomposed it is) is going to consume oxygen and lead to anaerobic decomposition. This helps to reduce iron, manganese, phosphorus and nitrogen to more available forms however these reactions proceed just fine within the top inch or two of the substrate. Stronger reduction processes deeper into the substrate produce sulfides and other toxic by-products. Some, but not all, aquatic plants are well adapted to growth in strong reducing environments such as marshes. Aquarium fishes are also sensitive to these by-products that may be released when you uproot plants.
6. Don't make the substrate too fertile. This is a common mistake of folks experimenting with soil substrates. You get a lot of plant growth doing this but you also risk potential alga problems such as BG cyanobacteria, green water (unicellular) algae, diatoms, or filament algae. Many products such as earthworm castings, potting soil, garden soil, or soil compost mixtures are quite fertile and if used, should probably be mixed with sand or silt or a sandy-clay mixture. An overly fertile substrate can release ammonia following submergence for a few weeks. Too much organic material decomposing also creates an oxygen demand; this can be a problem for the fish unless you have the plants producing plenty of oxygen. Ordinary topsoil (from the hilly meadow) usually contains a mixture of humus, sand, silt and clay, which is ideal for our purposes. The most important is that the soil provides iron and other micronutrients.
7. Give it time. Remember the transition period. Watch ammonia and oxygen levels.

I don't know how common micro nutrient toxicity is when using soils in substrates. Usually we only hear about the success stories. If anyone is interested in experiments in this direction, please contact Paul Krombholz or myself.

Another reason I like peat is that it releases humins into the water. These are low-grade toxins that inhibit the growth of bacteria and fungi. You need to know if the fish you keep prefer high or low levels of humins in their water. South American catfish, neons, discus and Anabantoids (Gouramis and Bettas) all prefer water with humins (so called black-water). Of course, tap water is not supposed to contain humins...

So long as the tap water is fit for drinking and doesn't contain an excess of chlorine or chloramine, I don't consider large, frequent water changes to be a problem. They are, in fact, very beneficial because they remove excess nutrients and low-grade toxins from your tank water. At the same time they might also help to replenish other nutrients in short supply.

If you are adding calcium carbonate to boost Ca content and as a pH buffer, you normally add this with the water change. The pH buffers already present in your tank water will dominate the pH even if your tap water is like rainwater so pH fluctuations should not be too much of a concern. Of course everyone keeping fish should have a pH test kit and be familiar with any pH problems with their tap water.

Fish are far more affected by bad water conditions than plants. It's probably a good idea to aerate the tank if you think the fish are short on oxygen.