I ordered substrate from aquariumplants.com--Igot the black--I have a 20 G long and they sent me 3 gallons. I started putting it in my tank and after about 2 gallons, it seemed too deep. How deep should the substrate be. I will have live plants over about 2/3rds of the tank. They will be low light plants--cryps java ferns and saggitaria. l