Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Organic means 3 things:
1) Derived from living matter (Plant or animal material)
2) Composed of molecules that contain at least one atom of carbon (Chemist's definition)
3) Some fuzzy definition of not-man-made.
Buying "Organic" potting soil is sort of buying into the hype of the word "Organic" (definition 3), but it does mean something in this use. Non-organic potting soil may have fertilizers, water retaining polymers and other things that we do not want in the aquarium.
Specifying a particular brand name gets us all onto the same page in that many other people have used that product for aquariums. Most often they are talking about Miracle Grow Organic Potting Soil. I am sure there are other materials out there.
"Topsoil" is another catch phrase that means different things to different people (including the manufacturer!). When you buy it in bags the goal is that you are buying the mineral portion of the soil (sand, silt and clay) with a minimum of organic matter (definition 1).
You can use garden soil. Just take it from a place that you know has been free of toxins for enough years that whatever may have been there has been broken down. A fairly active (micro and macro soil organisms is a good sign)
Then check that soil for the ratio of sand, silt and clay. If you want, I will post the instructions.
Add as much compost (organic matter, definition 1 and 3) as you want, but avoid the chunks that you can still identify. You want the finest, dust sized particles. If you make your own garden compost, I would screen it though hardware cloth to make sure you have only the smallest particles.
When you have the materials you want, mineralizing is optional; it is sort of like aging the materials faster than if they are submerged in the tank. You could put the material straight into the tank, then monitor the conditions. Some soils product a fair amount of ammonia for up to a month when they are first placed under water. This may be OK for plants (go ahead and plant the new set up) but is of course not good for livestock. The ammonia production is a good way to do the fishless cycle in the tank.