The substrate is the floor of the tank and the material where the aquarium plants will root. The most important factor is that the substrate should be a material that does not adversely affect the water conditions by raising or lowering pH and water hardness. You want a happy medium. Crushed coral or shells and certain kinds of gravel will create a high pH and high carbonate hardness, which is not good for your aquarium plants.
On the other hand, low pH and very soft water can cause root-rot which in turn will lead to algae growth. A pH of 6.8 to 7.2 is pretty much ideal, depending on the type of aquarium plants you select. Stick to natural substrates, avoid synthetic materials, gravel that is coated with epoxy or glass beads and colored ceramics.
Substrate material should be between 3 and 8 mm thick. Large granules will block root growth and smaller ones can actually crush the roots. It is recommended when starting the tank that you add the substrate in phases.
For example, mix the first layer of solid fertilizer, (remember - less is more) and place that in the tank. Then add as much as two additional layers with the finest granules on the top layer. Note that the additional layers do not contain fertilizer. Remember to use a bowl or plate on the bottom when filling with water in order to avoid churning the fertilizer up.
You also have to provide substrate at the proper depth for your aquarium plants. There are four groups of aquatic plants that can be classified by root type.
- Plants that don't need sand but attach their roots to rocks or wood such as Anubias, Microsorium, and Bolbitis.
- Plants with large rootstocks like Aponogeton and Nymphaea.
- Plants with long stems like Hygrophila and Rotala that have shallow roots.
- Plants like Cryptocoryne and Echinodorus that are deep-rooted.
To give you an idea of the depth of the substrate, you must consider the type of aquarium plants you are going to use. The deep-rooted ones will, of course, require the most depth. If they are planted in substrate that is not deep enough, the roots will become entangled and the aquarium plants will suffer from a lack of nutrients. The deep-rooted plants need at least a 6 cm deep substrate (2 to 3 inches).
Substrate materials for planted aquariums are abundant now, with two of my favorites being EcoComplete, and Fluorite Brown. Growth is lush and appears to be complete in terms of what the plants need. It does seem that there is a breaking period for the EcoComplete in terms of plant growth of about 2 to 3 weeks. In other words, the plants do not take off for that period of time.
If you are interested in doing a soil based substrate, get Ecology Of The Planted Aquarium by Diana Walstad; it is a complete book on how to plus some very scientific study on aquatic plants.