“I read all over your web site and only found one mention of coconut husk fiber as a substrate in a semi-aquatic tank. Is this a suitable substrate for growing plants, and does it sink in water? I’m interested in building an aqua-terrarium and perhaps putting in some frogs, if I can get the plants to grow properly, and I’d really like to have a sort of river in it, i.e. some water flowing over/through some kind of substrate, but I haven’t been able to figure out what kind of substrate to put in there. The tank is already set up with a juvenile snapper in it, but it’s just gravel (the snapping turtle digs up the plants and they end up floating around) right now. The snapper is going to have to go, sadly, but I’d like to begin planning for new residents. I’m afraid to put stuff in the water without knowing what it will do. I have a canister filter connected to an undergravel setup which, according to my water tests, is more or less keeping up with the turtle’s waste in conjunction with water changes. I don’t want to totally foul the water, but I’d really like some kind of soil-type substrate for plants to grow in. I’d appreciate any advice.”
Most soils spoil quickly when submerged or fully saturated with water. If you’re trying to grow aquatic plants use an inch of a substrate like fluorite or eco-complete (available at tropical fish stores), or mix these with small grade gravel. Most aquatic plants will grow in this without difficulty provided there is adequate light.
Some aquarium hobbyists also have success using organic topsoil underwater sealed with an inch or two or small grade gravel, however in a paludarium or semi-aquatic setup designed for amphibians I would hesitate to go with this method.
An alternative to this setup would be to carefully select the types of aquatic plants you use and only select species that can be mounted underwater on wood or rocks. Java fern or Anubias species work well when grown this way. Planted pieces of wood can then be placed directly on top of gravel or sand, or any other substrate you choose.
Also, as you mentioned, the snapping turtle will definitely have to go. They are best kept in a simple bare bottom setup without any substrate at all. This makes cleaning their aquarium easy, even if it is not as visually appealing.
For the land section of this new aqua-terrarium, coconut husk fiber or a substrate mixture based off of it can be used above the water line so that it stays moist but not wet. When soils become saturated with water many plants are prone to rot. Remember to use a layer of fiberglass screen or other divider to separate your drainage substrate of gravel from the substrate about in which you plan to grow plants.
More about this can be found in the article about Tropical Terrarium Design.