Don't want to scare you or anything but the tank is going to weight anywhere from 1100-1300lbs. Thats alot of weight and should be supported accordingly. Whats the support skeleton made out of that you screwed the ply to? And how are you planning on cutting out the doors? And are you planning on covering the plywood edge on the sides of the cabinet?
Any urethane sealer would work for the inside. Same stuff you laquer cabinets and floors with. There is also a primer that has a nice finish sheen from bullseye or the company that starts with a Z, forgot the name but its durable and acts like a 1 step primer/sealer.