How much substrate should there be in the tank? The LFS told me 1 to 1 and 1/2 lbs per gallon of water (I have a 20 g tank). It seemed like a lot but I bought 20 lbs. worth of gravel. Now I'm reading on another website that 1/4" of substrate is the absolute maximum. Also, I bought a Marineland 200 bio wheel and the impeller is really loud...are they all like this or is mine defective? Is there some way to make it more quiet? Thanks.