I build 2 training pools together in horizontal line. Tank can't leave right pool and enter left one. The door between them is only has space for aquanaut, I mean 1 tile only. Tanks require 2 tiles doors. Have you check the save file yet? And about training pools design - you can see what tank who stuck now looking at stairs on left pool what took 2x1 tiles of 2x2 there, so he can't move further even it manage to leave the right training pool. And this control device at the bottom of the pool took same 2x1 tiles of free 2x2. But device could be passed using way to right and top, but as I already said, stairs blocks the way.