Thanks Wok, we have allowed for drainage and a tap under the tank, and the floor is concrete, ground and polished, so no soggy carpets! This was actually for allergy purposes, ie no cat fur and dust mites living in the carpet.
I originally wanted the fishtank in the kitchen, where it would make up splashback behind the bench and under the cupboards, so you could sit there and stare at them while allegedly cooking tea. But the room behind it will be our plant room, and houses gas bottles and storage tanks for the underfloor heating, amongst other things, so that wouldn't work too well for the poor fishies.