This is part 2 of cleaning / water changing your fish tank. As you can see, you never remove the fish or all the substrate. The goal is to preserve the biological balance in the tank while taking out an amount of old water and detritus. This is what keeps your ammonia, nitrite and Nitrate levels balanced. The fish are only disturbed a little bit during the process. But they get used to it.
Drawn with pencil, colored with pens.