I just changed the water in the tank. I took the fish out, put them in fish bowls (betta in a little one), used the vacuum tubey thing to clean the gravel, and I took out about 3/4 the water. I put warm water in, put some of the stuff in that 'Makes tap water safe for fish!!' and replaced the filter. I let it run for a while, then put the fish back in. Good enough? I still haven't gone to petco, yet.