Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Hard to kill things in the substrate, but there is one way:
Put the substrate in a pan and dampen the substrate pretty well.
Bake it (250*F or about 125*C, temperature is not critical) until it is dry.
The boiling of the water, the steam, will kill a lot of things.
This REALLY STINKS! If there is ANY way of doing it outside (Bar-B-Que?) Do it.
You could try solarizing it, too. This means to put it in a low, wide box that has a glass lid, and is well sealed. Spread out the substrate so it is as thin as possible. Aim the glass lid at the sun. The sun, shining through the glass lid will heat the substrate enough to kill a lot of things, but maybe not everything.
Another option: Whatever you dip the plants in you might soak the substrate in, if you know that you can absolutely get the sterilant out of the substrate before putting it in the tank.
If you have one of the high CEC substrates this is tricky. I do not know all the materials these soils attract.
If you are using a sand or gravel that is easier: I would just soak these in bleach, then rinse with a double dose of dechlor. Air/sun dry to remove the rest of the chlorine.