I dont have AC either. When I had t5HO lights the temp in my tank would also run really warm in the summer..but that was with a 110g. I had a little fan that I would point towards the water and get evaporative cooling to drop the temp a bit. However with 2.5 gallons you'd lose a lot of water pretty quick. Also I wouldnt use the ice cube method directly in that tank. I was wondering...could you sit that 2.5 gallon in a pan of cooler water. Maybe not packed with ice cubes but maybe just one or two? Id do it gradually starting with just cold water...see how that works, then add an ice cube. As far as heat tolerant fish, I used to keep cardinal tetras with discus. Discus like warmer water, so you may look up tank mates for discus. But with 2.5 gallon im not sure that would be appropriate for cardinal tetras either.