Tank size

[SanderB]

The bigger the tank the better. But and some floating plants or plants that reach the surface, so they can hang and breath.

[rodsboys]

That is definately not too deep. Especially if you keep the level down a bit to help prevent escapes in addition to a well sealed lid. They can and do hold their breath for quite some time and I doubt an extra split second of swimming up is going to kill them.