18. Natural River Rock - (manufacturer unknown) - very hard calcined clay; light beige; resembles aquarium gravel; 2 to 5mm in size; no organic matter

19. Flourite - Seachem, USA - fracted, stable clay; very hard; multi-colored (brown, red-brown, black); resembles aquarium gravel; no organic matter

20. South Carolina topsoil - South Carolina, USA - sand and silt; large amounts of dark humus; soft and powdery; orange-brown; will float and cloud water

21. Play sand - Lowe's, USA - clean, no visible trash; white and clear; 1mm in size; no organic matter.

22. CaribSea Tropic Isle Laterite -CaribSea, USA - very hard lateric rock; deep brown-red; resembles large aquarium gravel; 2-10mm in size; no organic matter

23. Onyx gravel - Seachem, USA -naturally source-fracted substance; appears to be porous clay or rock; very hard; light and dark gray; resembles large aquarium gravel; 210mm in size; no organic matter

24. Tetra Initial Sticks - Tetra Products -substrate additive; medium hard, will crumble; dark gray; 5 to10mm pellets; large amount (>20%) of organic matter; will float and cloud water

25. India local laterite - Northern Nune, India - powdery; deep brick-red; silt-

2 mm in size; very small amount of organic matter; will cloud water at least temporarily of those individual decisions. There are good arguments on both sides. Some people allow fish and food wastes to remain, providing 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 your tank needs a total breakdown.

Life Cycles

As with most things, there is a break-in period for substrates. Newly planted tanks may take a few weeks or several months to stabilize. Ammonium, nitrite, and nitrate levels will bounce around until the bacteria are established. Meanwhile the plants will then start establishing themselves and your tank will approach a balanced condition.

As with potted house plants nutrients can become exhausted over a period of time. The planted aquarium also has a life span, so you may need to replace nutrients or the whole substrate. Reverse undergravel filters/undergravel filters (RUGFs/UGFs) and heating cables can affect the 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 increases life span by making a more efficient bed, or decreases life span by using up the nutrients more rapidly.

Heating cables can be used with most substrate choices, but RUGFs/UGFs need to be used with well-consolidated substrates such as hard, calcined clays or lateric rock, and not with laterite dust. Heating cable flow is determined by the amount of heat being used; more heat causes faster flow. The right wattage should be used to get a slow, gentle flow. Undergravel filters present the same issue, again slow and gentle. Too much flow may increase unwanted nutrient levels in the water column.


One analyst did all the testing. The same instruments were used for all samples, and testing took approximately one week to complete. This is important since methodologies used by individuals may vary. I initiated the research with a request for testing materials via The Aquatic Plants Digest (http:// index.cgi). 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, a very good representation of available candidates.

Testing consisted of measuring the soils' pH, total leachable metals, and cation exchange capacity (CEC). Soil pH is important because it can influence the pH of your aquarium. Total metals are bound in crystalline structures. Materials release their metals at different rates, depending on the details of these structures. The hard, calcined clays have the ability to retain nutrients longer than the soft, moldable clays.

The CEC determination helps gardeners know which substances are more efficient at nutrient binding. CEC measures 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 substance. As long as a material has a measurable CEC, it should work well in an aquarium's fertile environment.

The pH determination was done with equal amounts of test material 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. Total metals were analyzed on TJA61E inductive coupled plasma (ICP) instrument. Acid digestion of samples was done according to EPA SW-846 Method 3050A. CEC determinations were done by Method 9081A of EPA SW-846. CEC extractions were also analyzed on the ICP. Samples were analyzed in their original forms. Care was taken to analyze them as they would be used by aquarists. Note that crushing would increase the surface area and may change the parameter values here.

Data Analysis

The table on pages 20 and 21 contains the data from the chemical analyses of the substrates tested.


Aquatic plants demand a critical supply of iron for proper development. The search for iron sometimes dictates the substrate choices we make. Macro-nutrients 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 others 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 gets an honorable mention for total mineral content.

A majority of aquarists prefer a neutral to acidic substrate. Bacteria often have peak productivity at a pH of about 5.5. Plants best use macronutri-ents at a neutral pH, while micronutri-ents work best at a lower pH.

Even though collected a world apart, the two redart clays were similar and had alkaline pHs. The preglacial Ontario clay had a lot of calcium carbonate (CaCO3) and its pH reflects this. It was very different from the Finish clay, which resembled it in color and composition. The Onyx gravel reinforced its buffering claim with a high pH, but the big surprise came 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 I 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 was a close second to the Tetra Sticks. All other samples had close to neutral pH values.

Cation Exchange Capacity (CEC)

Some gardeners swear by CEC values, while others grow aquatic plants with disregard for CEC. 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-exchange medium and has good mineral concentrations. The Tetra Sticks, with their 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 you notice. The numbers provide the aquatic gardening community an invaluable resource. The table can help narrow 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 his 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 used if set up and maintained properly.

About the Author

Jamie S. Johnson is a trace metals chemist and has been in the aquarium hobby for more than 15 years. He has been growing aquatic plants for about five years. Jamie is a member of the Aquatic Gardeners Association and The Aquatic Plant Digest. He can be reached via email at [email protected]

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