Even though your heater has a thermometer has a temp gauge on it get a good digital thermometer we had our heater set at 78 degrees it turns out that was high by 10 degrees luckily it was when our tank was cycling so it wasnt that big of deal how ever we didnt know it til after we added CF and one died its symptoms were exactly as you described when ours died just one was gotten by the emerald crab but thats when we started investigating and found the heat was way to high now our heater is set at 72 and the tank runs between 77.6 and78.3 no problems since