What ype of "sand". No play sand please. If so remove it. You want aragonite, sugarfine, or south down. Don't get crushed coral either as it is a waste trap. Will make your water sour. Well as long as you tend to the filter it won't hurt running it on a small tank. If you plan to go "natural" you will need lot's of quality porous live rock. About 15lbs would be good. I have a small filter on our 7g nano to provide currents for the tank. It gets cleaned weekly. It isn't for biological filtration, only mech. If you let the wastes sit in it to long they break down and repollute the tank. A great idea for larger HOT filters is to cut some of the veins off the impeller to slow it down. Add a layer of mud and aragonite to the bottom of it and add cheato macroalgae. This makes a mini fuge on the tank providing for nice filtration and a place for microfauna to grow. It can take as long as a week for the substrate to settle out. I turn off my pumps but another person once told me that keeping them on will settle a tank faster. I couldn't see why but he told me it causes the particles to bind into heavier pieces. After 3 days I decided to turn off the pumps until it settled.