Even with the added water you are still going to have big swings in your water temps. If you are going to keep the fish ouside and control the temps, you need to consider insulating the sides of the pool and get some kind of cover for the top. How big of a pool is it? abover ground pools genarally are thousands of gallons, and even if you are only filling it up a 1/4 of the way, 2 aquarium heaters are just not going to make a difference. As stated earlier, the tropical fish need constant temps and will suffer the same fate as your ID Shark if you can't stabilize the water temperature.