I've used a number of materials as a bottom foundation for my freshwater planted aquariums. In some, I've used only epoxy-coated aquarium gravel. In others, I've used a mixture of potting soil, clay, red flint, various types of aquarium gravel, Volcanite and EcoComplete.
The aquarium pictured here is an established 20 gallon "wide" with very bright lights, RO water and a substrate consisting of 60% Schultz MoisturePlus Potting Mix from ACE Hardware and 40% red flint from Red Flint Sand and Gravel. The aquarium water is clear. The plants grow well and some are prolific, requiring weekly pruning. This particular aquarium contains WAG platy and a couple of clown loaches. Click on image to enlarge.
I've watched this aquarium grow for several months. When I first set it up, I had no intention of keeping many plants. It was meant as a holding place for cuttings from my other aquariums. To my surprise, the substrate mix of soil and flint has proven more than adequate.
Shown here, the same aquarium with an infestation of Water Lettuce. This shows that the aquarium has enough nutrients (substrate and added liquid ferts) to accommodate fast plant growth. I keep a few Water Lettuce specimens but I don't allow it to grow this thick because it prevents light from reaching other plants.
In the beginning, there was a surge of turbidity which dissipated over time. This initial cloudiness might have been caused by finer particles of dust leaching into the water column. With regular water changes and the progressive growth of plants, the water cleared up.
Currently, there is a slight growth of hair algae. However, I strongly believe that this is an imbalance caused by liquid fertilizers, not the substrate. I have another less established 20 gallon tank that has much more flint and soil per square inch than this one, and there is no hair algae whatsoever.
Picture on left shows aquarium at time of writing. Some hair algae has started growing on smaller plants near the bottom. I am trying to control this by reducing Iron. I am also experimenting with commercial products.
I would conclude with these observations that a substrate made up of potting soil and flint is a cheap and adequate alternative to other expensive commercially made planted tank substrates. I would not hesitate to build another aquarium with a soil and flint bottom.
Benefits of a soil and flint substrate:
#1. Inexpensive.
#2. Provides much needed nutrients for plants that feed through their roots, such as Swords.
#3. Water may not need extra liquid fertilizers.
#4. Overall plant growth seems to benefit.
Cons of a soil and flint substrate:
#1. May leach fine particles that cloud up water after initial setup.
#2. May promote some algae growth over more inert types of substrate due to excess nutrient levels leaching from the soil. I'm currently testing to see if I can control this by reducing certain nutrients in the water column, such as iron chelate.
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