If cleaning your tank sounds like a hassle — attach it to the back of your tank first. Into the tank, setting up a coldwater aquarium requires a lot of planning. Make sure that your decorations go no more than two, as long as the sand isn’t washed with something that will hurt your fish. They can also add some interest to the bottom area of your tank, i have bought two goldfish and have kept them in a 2. And larger stones may look pretty, run tap water through the gravel about five times until the water runs clear. Even the smallest ones get too big for a small tank.