Ok, what's going on? Do you have a way to test your water? If so what are the ammonia, nitrites, and nitrates? Do you use anything to dechlorinate your water? What is it? Tell me what you have been doing and what has changed
Check your water temp and make sure it is within your fish's parameters. Also, did you quarantine the new fish? they could have something from the store. Finally, did you rinse the heater? It could have had some chemical on it from packaging or production.