I had an empty ten gallon with nothing in it, except it was cycled and had water running through a filter. I found some spare black gravel in my house, and I decided to try some stuff with it for spare plants or maybe just a plant grow out tank. I put about 1 1/2 inches of natural bentonite clay kitty litter, abt. 2 cm of dirt, and lastly capped with the gravel. For the most part of today it was pure black water, but I added some 25 micron filter media and now it is crystal clear!i haven't added any plants yet, because I want to be sure I didn't make a death trap. Here it is!