I use limo tint from an auto parts store to dim my lights, it works great and you just lay it down under your light. 3 adult Red Belly's are gonna need at least a 75 gallon and probably should get a 90, they are territorial so you want to make sure they can all have there own little space to hover at the bottom. They should be ok for a very short time in the 20, but I would say you should upgrade as soon as possible. Good Luck