The problem with gravel cleaning on small tanks is that the water is removed too fast before you can vacuum the gravel. However, if you use a small submersible 60 gph
pump, some tubing, a pill bottle and floss you can clean the gravel and the water goes back into the tank. Here is a pump...
Simply add about 18" of tubing, cut the bottom off the pill bottle and drill a hole in the cap for the tubing. Put the pump on the inside glass and some floss in the pill bottle and turn it on. Have extra floss ready to sway out as is clogs quickly. Works like a champ.