How much substrate is a good amounts for a ten gallon tank? (10 x 20 inches) I'm thinking of switching to black gravel but I'm not sure how much to get. I could get two bags of 5 lb gravel for 5 bucks each, or I could get 25 pounds for 20 bucks. But I don't just wanna spend the extra ten bucks just for those savings, I have other things to buy too So please tell me how much is enough, by enough I mean covering the whole bottom, a few layers (like enough to hold most plants down)