Puh, what a mission!
Had ants living in the tiny gap between two tanks. Obviously you can't just use flyspray in your fish room so I tried some "NO Ants Gel Bait" and after 6 weeks they were gone. All that is left is a pile of dead ants under the racks that got chucked out by their (back then still alive) mates. Together with dust, fish food and a million hair of our Labrador it was quite a mess.
So I gave the whole fish room a good clean, scrubbed the walls and the floor and it looks brand new again :)