umm you could try to make a larger tank??? Or cover the tank so the fishies don't swim out
Or you could try this :::::::::::::::::i'm not sure if this will help but i have been working with angel fish and here is what i found....
you have to set your bounded box first. Then place your fish inside the box. AFTER it is in the box select move within bounded box script from the advanced window.
Parameter 1 is left to right movement
Parameter 2 is up and down movement
Parameter 3 is front and back (depth)
Paramters 4, 5, and 6 don't seem to do anything, at least not for angelfish. I don't know if this works for all fish or not. Try setting one parameter at a time while leaving all the others blank so you can see what they do first.
Hope this helps.