as long as the tank's not deep enough that they can't reach the top in more than a "bound" or two they'll be fine. if you have an airstone in it you'll notice they like to ride the bubbles to the surface and then parachute back down.
So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth,
And pray that there's intelligent life somewhere up in space,
'Cause there's bugger-all down here on Earth.
Loki/Grendel/Gail/Beowulf/Camilla/River/Bathory/Arwen/Lafayette/Deb/Jack Skellington (soon)