Substrates for the Planted Aquarium
By Jamie S. Johnson
Aquatic plants, water chemistry, and cichlids
summer 2000

There are a wide variety of substrates for use in today's planted aquarium. Due to the growth and popularity of aquatic plants, new products are being introduced all the time. This is good for us gardeners, but it only adds to the confusion of which type is best. These new varieties, along with the old standards, has given us many different choices. Some people stick with the proven recipes, while others experiment with new and sometimes untested ideas. A lot of real world data exists to substantiate the viability of commercial substrates and additives, as well as homemade peat, vermiculite, or soil blends. It's all up to the needs of the individual aquarist. Some like the simplicity of commercial products while others enjoy preparing the substrate they believe to be most productive.

No single substrate that can be labeled as the best, but there are ones which perform better than others. Many factors affect the type of substrate needed: types of plants, appearance, growth rate, maintenance, inhabitants, chemical properties, budget, and availability. As you can see, there are several parameters that go into deciding the optimal substrate. The purpose of this substrate analysis and overview is to help narrow down some of those parameters, especially the physical and chemical properties.

The substrate serves many different purposes in the planted tank, probably more than in any other type of aquaria. It provides a place where mineral and organic nutrients are stored. These nutrients are released to root-feeding plants as needed. It also provides a bed for the growth of beneficial bacteria. These bacteria are responsible for breaking down wastes. They also are responsible for the mechanisms that cause reduction reactions on the nutrients, making them available for uptake by the plants. Iron, along with other nutrients, needs to be in the reduced state to be utilized by the plants. Reduction turns the commonly found ferric iron (Fe+3) into ferrous iron (Fe+2). The negatively charged sites of the substrate attract and hold the positive ions until needed by the plant's roots.

The bacteria also breakdown fish and plant wastes, as well as extra food. When new tanks are set up, the bacteria are just beginning to establish themselves and this is usually what causes the phenomenon "new tank syndrome". The aquarist may experience high ammonia and nitrite spikes until the tank settles in. It may be beneficial to seed your new tank with some gravel from another trustworthy tank. This will give the bacteria a jump start.
As well as being a good anchoring medium, a substrate must be aesthetically pleasing. Fish and plant colors will appear more deep and rich with a dark substrate. This is good look for a soft water, Amazonian aquascape. A tank with a top layer of sand usually resembles a shallow shoal, bright and alive. Fish may be more timid with the washed-out bottom color.
Commercial, as well as prepared substrates must have the correct size granules. Too large and waste will settle down deep, clogging the substrate from nutrient exchange. Too small and it might have the tendency to settle and compact. A compacted substrate will not allow for the growth of small, delicate roots. It would also impede the flow of nutrients throughout the bed. Eventually, in both cases, growth would slow and your plants would suffer.

Another concern would be the buoyancy of the substrate, it should sink, and stay sunk. If not, cover with a top dressing of sand or gravel. Materials like pumice, peat, humus, and vermiculite tend to float if given the chance. Boiling these before application will saturate them, helping keep them controllable until covering.
Try to avoid using fine-granulated sands. Choose the largest grade available. Beach sand should also be avoided. Gravel size should be 2-5mm, and luckily these are the most popular sizes. The gravel and sand need to be chemically inert. This will insure the pH and other water parameters aren't affected by the substrate. Before application, add a drop of hydrochloric acid to the material in question. If it fizzes or foams, do not use it, or be aware it may alter your water chemistry.

Sand and gravel need to be washed thoroughly before use to remove the dust and trash. Make sure your substrate does not contain shells. They will increase the hardness and alkalinity over time. If a commercial product is being used, follow their preparation instructions. You can even experiment, there are endless possibilities.

Calcined clays, lateric rock, and zeolite, can be used as complete substrate beds or mixed up to fifty percent with other products. Plain gravel makes a good mixer, but should be avoided as a stand-along substrate. Be sure to thoroughly rinse these products, they can contain a large amount of fine dust that can initially cloud the water and settle on your plants. Lateric soils, redart clays, and soils need to be mixed with gravel and put in the lower third of the substrate. These types cannot be rinsed before-hand, and will easily mix into the water column if left too close to the surface. Collected soils need to be sterilized in an oven at 200ºF for one hour and then sifted to provide the highest quality soil. Be careful not to collect near heavily traveled areas or areas that could be easily contaminated. Aquariums are closed systems, so quality is paramount. Peat, vermiculite, and other additives would also be mixed in the lower layer. Cover the lower layers with a top layer of gravel or sand. You are now ready to plant.

Tanks are most appealing if the substrate is terraced from back to front. A minimum depth of 3" in the front to a minimum of 5-6" in the back is best. This allows for the entire surface of the substrate to be viewed, from the small foreground plants (glosso and chain swords) to the larger, heavy feeders (swords and crypts). It's up to the individual to decide on the final look, but remember to provide a good depth for root development.
If substrate heating cables are going to be used, a small base (1/2-1") is applied for the cables to rest on. They need to be placed in the correct orientation for the optimal affect, flat with clearance on all sides. Cover and complete the substrate as normal.

Regardless of what substrate you decide on, problems can arise. They maybe built-in problems, too rich or organic, or they may gradually appear, low in nutrients or compactness. The built-in problems can be controlled, to an extent. Peat, manure, leaf debris; all can be used, but in moderate amounts. With the advances in today's fertilizers, manure's disadvantages may outweigh it's advantages. Peat and leaf debris also decompose to form noxious, low pH environments. Laterites and clays are rich in minerals, but not organics. These minerals are stored within the substrate and are not as readily available to the plants as the organic are. High mineral concentrations rarely cause problems, but the absence of certain ones will. Too little or too much of anything is bad. That's why it's important to be aware what minerals are present and in what concentration. Nutrients can be amended to the substrate to correct deficiencies. Clay balls can be moistened and baked at 250ºF until hard, then inserted under the plants that show problems or are heavy feeders. Mulm can't always provide the nutrients needed for a fast-growing tank, but time-released fertilizers (Osmocote) or plant spike (Jobes) can keep things in balance. They need to be low in phosphorus (middle number of N-P-K), as not to promote algae growth if leached from the substrate.

Compactness may be experienced somewhere down the road for an aged tank. The plant roots alone could amass to cause problems, on top of physical compacting. Vacuuming the gravel LIGHTLY will help to prevent compactness. It will also give the tank a cleaner appearance. Care must be taken not to disturb additives or fertilizers. Mulm is removed and more oxygen is supplied to the roots. Vacuuming is another one of those individual decisions. There are good arguments on both sides. Some people allow the fish and food wastes to remain, proving food for snails, bacteria, and plants. However, a clogged substrate is not a healthy substrate, so a periodic light vacuuming may not be a bad idea. Most problems can be resolved before a total breakdown is needed.

As with most things, there is a break-in period for substrates. Newly planted tanks may take a few weeks or several months to become stable. Ammonium, nitrite, and nitrate levels will bounce around until the bacteria are established. They will allow more nutrients to become available to the plants. The plants will then start establishing themselves and a balanced tank can be achieved. As with potted house plants, the nutrients can become exhausted over a period of time. The planted aquarium also has a life span, so nutrients need to be replaced or the substrate replaced.

RUGFs/UGFs (Reverse underground filters/underground filters) and heating cables can manipulate the normal life span of the substrate. They cause a greater flow of nutrient water through the bed, improving nutrient exchange rates. It is not known if this causes an increase of life span by making a more efficient bed, or a decrease of life span by using up the nutrients more rapidly. A thousand arguments have been raised and debated, but it's still up to the aquarist. Heating cables can be used with most substrate choices, but RUGFs/UGFs need to be used with hard, calcined clays or lateric rock only. Heating cable flow is determined by the amount of heat being used, hotter causing faster flows. The right wattage should be used to get a slow, gentle flow. Underground filters are the about the same, higher flow rates having faster flows. Again, slow and gentle. Too much flow may increase unwanted nutrient levels in the water column.

All testing was done by one analyst. All samples were analyzed on the same instruments and testing took approximately one week to complete. This was important, since methodologies used by individuals may vary. I initiated the research with a request for testing materials via The Aquatic Plants Digest. I had several substrates of my own to start, but tested a total of 25, consisting of commercial products, local soils/clays, and homemade blends. I feel it is a very good representation of what's available.

Testing consisted of soil pH's, total leachable metals, and cation exchange capacity (CEC). Soil pH is important because it can show the chemical possibilities of your substrate. It's chemical properties could alter the surrounding water column. That is not the same for the total metals. These analytes are bound in the crystalline structures. Materials release their metals at different rates, depending on the make-up. The hard, calcined clays have the ability to retain nutrients longer than the soft, moldable clays.

The CEC determination helps us gardeners know which substances are more efficient at nutrient binding. CEC is a reversible chemical reaction between a solid and a fluid in which ions may be interchanged from one substance to another. The values are expressed in milliequivalents per 100g and are the total sum of exchangeable cations of a soil. As long as a material has a measurable CEC, it should work well in an aquaria's fertile environment.

The pH determination was done with equal amounts of soil and deionized water. The samples were shaken to mix thoroughly and allowed to settle before testing. Testing was done with an Orion 720A pH/Conductivity meter. The total metals were analyzed on an inductive coupled plasma (ICP) instrument. Acid digestion of the samples was done according to EPA SW-846 Method 3050A. The CEC determinations were done by Method 9081A of EPA SW-846. CEC extractions were also analyzed on the ICP. Samples were all analyzed in their original forms. Care was taken to analyze them as they would be utilized by the aquarist. Note that crushing would increase the surface area and may change the parameter values given here.

Iron/Minerals

Aquatic plants demand a good supply of iron for proper development. The search for iron sometimes decides what substrate choices we make. Macronutrients like calcium, magnesium, and potassium are also important in plant growth. Zinc, copper, manganese, etc. must be present in small amounts to provide essential micronutrients. First Layer Laterite has the highest amount of iron, a strong 11.8%. The local Indian laterite was right behind, with 11.4%. These values are a good approximation of total amounts. All the lateric soils have good iron content. The redart and natural clays had medium iron content. All other had medium to low amounts. If iron levels are low in your substrate, just amend it with clay or laterite balls. The Yolo loam/vermiculite blend has got to get an honorable mention for total mineral content.

pH

A neutral to acidic substrate is the preference for the majority of aquarists. An important thing to remember, most bacteria are most productive at a pH of approximately 5.5. Macronutrients are best utilized by the plants at a neutral pH, while micronutrients are best at low pHs. The two redart clays had alkaline pH's and were comparably similar, even though a world apart. The preglacial Ontario clay had a large amount of calcium carbonate (CaCO3) and it's pH corresponds to this. It was very different from the Finish clay, which resembled it in color and composition. The Onyx gravel reinforced it's buffering claim with a high pH, but the big surprise can from the CaribSea Laterite, which posted an even higher pH of 9.8. It does not seem to contain any calcium carbonate (CaCO3).

The organic matter present in some of the samples would seem to dictate low pH values, and that is exactly what was found. The Tetra Initial Sticks had the lowest pH. They would work on new set-ups by getting an acidic bed started, allowing for a quicker exchange of nutrients. Nitrifying bacteria work better in these low pH environments. AquaTerra also contained a large amount of organic matter, and it was a close second to the Tetra Sticks. All other samples had pretty common pH values.

Cation Exchange Capacity (CEC)

Some gardeners swear by CEC values, while others grow aquatic plants with nothing but blind faith. It is interesting to know the actual parameters of each substrate, but I've seen plants grown in plain sand and gravel. The samples that lacked at least some organic matter or clay, had slightly lower CECs. The big surprises here were Terralit and Tetra Initial Sticks. The zeolite composition of Terralit provides an excellent ion exchanging medium and has good mineral concentrations. The Tetra Sticks, with it's high organic and mineral content, also had a high CEC. The sandy samples had very poor CECs.

The longer you look at the data tables, the more observations you begin to notice. I think the data has provided the aquatic gardening community an invaluable source of knowledge. Hopefully it can help narrow the decision making and clear up some of the confusion associated with substrate choices.
I hope this analysis and overview has provided a good starting point for a successful planted aquarium. Regardless of data or arguments, it is still up to the individual to decide what's best for their needs. I have grown plants in sand, gravel, Flourite, and litter. All substrates gave good growth, given all other parameters were optimal. It's true some were more attractive, had higher iron levels, or contained more organic matter, but all can be utilized if set up and maintained properly.