Aquarium Frontiers On-Line: Aquatic Horticulture

AQUATIC HORTICULTURE

BY KAREN RANDALL

Substrate Options for the Planted Tank — Part 1

There are many different materials available for use as the substrate in a planted tank, and many different views on what constitutes the best combination. Some people don’t put much thought into their substrate and are quite successful anyway. Some, particularly those who are most interested in the ornamental beauty of an aquatic garden, prefer to follow a tried and true “recipe” that has been proven to produce good results under a number of conditions. Still others enjoy experimenting. They prepare elaborate substrates to try to promote what they consider to be optimal conditions for plant growth and are not disappointed with a failure now and then.

All of these approaches are valid. The important thing to do before making a decision on substrate is to decide what is important to you, and then choose the materials accordingly. First, let’s look at the properties of good substrate.

A good substrate must be of a consistency that is easily penetrated by roots. It must be fine enough to allow minute roots to push through, but at the same time, it should not be so fine that it easily compacts and impedes root progress in the process. It should be composed of substances that will not affect the hardness or pH of the water, and it should not have sharp edges that can damage roots, or the sensitive barbels of bottom-dwelling fish.

Quality substrates should not contain any substances that are toxic to either fish or plants, and should be able to provide all the nutrients needed by root-feeding plants on a long-term, sustainable basis. This can be done either through adding nutrients directly to the substrate or by relying on the substrate’s “cation exchange capacity” (CEC) — its ability to chemically capture and hold nutrients in the substrate where they can be accessed by plant roots. Now, let’s take a look at some of the materials used by aquatic gardeners in their substrates and discuss the benefits and drawbacks of each.

Gravel

The main ingredient in most substrates chosen by aquatic gardeners is gravel, and there are good reasons for this. pH-inert gravel is an inexpensive and readily available material in most parts of the country. It is available in a variety of sizes and a range of attractive natural colors. It is easily cleaned, and as long as the grain size is small enough, it will help keep other substances down deep in the substrate and out of the water column. The size gravel that seems to work best is between 1 and 3 millimeters in diameter. This corresponds with aquarium gravels graded #1 through #3. Many people find less expensive sources of gravel by visiting gravel pits, buying appropriately sized sand blasting sand, “traction sand” (if you live in snowy areas) or chick grit sold at farm and feed stores.

Whatever gravel you choose, make sure it is chemically inert. You can test for this by putting a drop of hydrochloric or muriatic acid on the gravel. If it foams, it should not be used in a planted tank.

Next, look at the gravel carefully. Each grain should have rounded edges that will not damage tender roots or the barbels of bottom-feeding fish. The smaller sizes are preferable if you plan to use small rooted foreground plants, while sizes smaller than #3 will fall through the slits in an undergravel filter. Whatever gravel you use, plan on using a minimum of about 3 inches of gravel. This can be increased to as much as 6 inches in the rear of the tank where large plants with heavy root systems (like Echinodorus and Anubias species) will be planted.

A few people use very fine sand to “cap” their substrate in an effort to keep nutrient-rich materials from seeping into the water column. I would not advise this method for a couple of reasons. First, sand packs tightly enough that there is a real risk of seriously anaerobic conditions developing, particularly if there are large amounts of organic material below. Second, sand is so lightweight that it is very difficult to siphon mulm from the surface without removing at least some sand along with it. Over time, you wear away your top layer, exposing the nutrient-rich layer that you meant to isolate.

Gravel Additives

Because many of the plants we use in our aquariums are amphibious and draw a substantial amount of nutrition through their roots, the substrate must be rich enough to support them. A lot of old timers in the hobby depend on fish wastes to feed their plants. The problem with this is that it can only work in an older tank that has had time to build up a good layer of detritus in the substrate. If used exclusively, it means that you need to wait for quite a while before adding plants to a tank in order to allow the substrate to become fertile. During this period of time the nutrient levels are also rising in the water, which, in the presence of high light levels, can lead to algae problems.

If there is some reason that you feel that you must use a plain gravel substrate in a tank, but still want to enjoy live plants, make sure you choose plants that are adapted to take their nutrition directly from the water column. Water sprite (Ceratopteris thalictroides), Salvinia Java fern (Microsorium pteropus), Java moss (Vesicularia dubyana) and hornwort (Ceratophyllum spp.) are some species that come to mind. Most aquatic gardeners prefer to start with a heavily planted tank to head off algae problems. For this reason, it makes sense to start with a substrate that is capable of feeding the plants from day one. This can be accomplished in a number of ways.

Clays

My preferred method for the novice aquatic gardener is to use commercially available laterite in the substrate. Laterite is a non-organic, iron-rich tropical clay. It serves several purposes in the substrate. It provides the plants with an initial source of iron and other trace elements. More importantly, it serves as an ongoing chemical attachment point for trace elements circulating through the substrate. Finally, this same chemical binding quality means that it “captures” phosphate in the substrate, keeping it down where rooted plants can get to it and use it, but where it is unavailable for algae use. It is important to recognize that not any red clay-like earth is not laterite, and even true laterite can vary a lot depending on the source.

Laterite is a highly weathered substance found only in tropical (or once-tropical) areas. Reputable companies put a great deal of time and effort into supplying a very reliable product that is safe, effective and varies little from one package to the next. One company that has been producing an excellent laterite product for a number of years is Dupla. There are several man-made laterite substitutes on the market, and some of these are a very acceptable as well.

Some people try using local red clay soils or red clays meant for pottery in their aquariums with varying results. Depending on the chemical makeup of the specific substance in question, this can work well or be a disaster. Likewise, a few people advocate the use of kitty litter as a substrate for planted tanks. As long as it is a plain clay kitty litter, with no added color, deodorants, clumping ingredients and so on, it is probably better than a plain gravel substrate. But, kitty litter is processed for use in cat pans. There is no quality control to guarantee that there will be no substances in the clay that could be harmful in an aquarium. If you want to play with these “non-aquarium” substances, remember that experimentation can be fun. But realize that this is exactly what you are doing, and don’t be upset if things don’t turn out as you hoped!

One point to understand about laterite substrates is that they are not meant, by themselves, to meet all the substrate needs of your plants. Nutrients will still need to be added, either through regular liquid supplementation that can be drawn down into the substrate, or through the addition of nutrients directly into the substrate in solid form. Many aquatic gardeners find it beneficial to use plant food tablets to give an extra boost to the substrate, either initially or on an ongoing basis. This is a particularly useful practice in tanks without some form of substrate circulation. It is very important to know whether these tablets contain phosphate and/or nitrate. In an intense light environment, excess nitrate or phosphate can cause severe algae problems, so tablets containing theses substances should be used only in tanks with a known deficiency in these areas.

For tanks that are not deficient in macronutrients, but where the aquarist feels that supplementation of trace elements at the root level would be beneficial, there are several companies that produce plant food tablets specifically for aquarium use that contain neither phosphate nor nitrate. My personal favorite for times when I feel that my tanks are in need of macro nutrients is Jobes Plant Food Spikes for Lush Ferns and Palms. These come with an nitrate-phosphorus-potassium (N-P-K) ratio of 16-2-6, which is the lowest phosphorus ratio I’ve found in any macro nutrient supplement. This is important because phosphorus is the last nutrient likely to become limited in an aquarium housing a population of fish.

Soils

There is no doubt that most plants, whether terrestrial or aquatic, were meant to grow in soil. A good soil substrate can supply all the nutrients needed for plant growth for an extended period of time. But, soil is not without its drawbacks. Soil is very variable both in terms of the mineral content and the organic content. Some people like to mix organic material into the lower levels of the substrate. This can be peat moss, potting soil, garden soil or earthworm castings. In the old days, people even used to use rabbit droppings! If properly managed this can work well, particularly in a low-light environment.

In a strongly lit tank, algae can be a serious problem with these organic substrate materials. There are other potential problems as well. All of these substances are extremely messy if they are allowed to enter the water column. Peat moss and potting soil (which often has a lot of peat in it) can decompose and rot in a compacted substrate. If you choose to work with either of these products be sure to purchase only those that do not contain fertilizers or other additives like perlite (which floats).

Garden (and other) soils may contain toxins of many sorts. It is a matter of luck and experimentation to see if what you have in your yard can be safely and effectively used in the aquarium. If you have good soil, this is usually a better substrate additive than peat or potting soil because of its lower organic component. The best place to dig soil for an aquarium is far away from any chemically fertilized lawns or gardens or any area where pesticides have been used. Scrape away any undecomposed leaf litter on the surface and use the soil from below. I strongly suggest that any soil substrates, whether they are purchased or dug from the yard, be tested in small trial aquariums for at least six months before you place them in the bottom of the large display tank in your living room for the first time!

Finally, some people have recently been playing around with vermiculite, which is crushed, soaked and crushed again so that it no longer floats. This is an inexpensive, inert substance with a very good cation exchange capacity. It does not initially contain the iron and other trace minerals that good laterite does, but with time and regular trace element supplementation in the tank, it should create a very good rooting medium eventually. The downside? I haven’t tried it, but the process of getting it to sink sounds tedious, and the look of vermiculite that does find its way to the surface of the substrate may be objectionable to some people. Next month we take a look at “the aging substrate.”