If I'm understanding correctly, you are looking for a way to keep your temp low rather than just a one time solution to a busted heater? I think the ice would work as an emergency method to lower temperature, and could work on a regular basis if you are always there to closely monitor the temp. The problem I see is that depending on ice to keep the day to day temp correct leaves you open to drastic temperature swings which may well be worse than just the high temp. I don't see how you can maintain a consistent temperature in the tank unless you are always at home to watch it and add/remove ice as needed. You may be better off looking at an air-conditioning unit for the room or a chiller for the tank to provide more long term consistency.