I spent many days pondering the problem. If the tank were small I could just fill it with cleaner, shake it around some, wash, rinse, repeat until clean. Of course it's a 20 gal tank, I am not the Hulk, can't just shake this tank. So I figured I could build a rotisserie for the tank, get a little electric motor to turn it and slosh the cleaner around until it's all clean.
Step one, build box around the tank.
I purchased one 4*8 sheet of Advantech 3/4" subfloor.
