How big is the tank and what is your water changing schedule?
Keep in mind that in uncycled tanks, you must do frequent 100% water changes as a part of your routine. A one gallon tank should get a 100% change every other day, a two gallon tank should get one every 3-4 days. I don't recommend those cheap 7.5W heaters--they don't keep the temperature consistent since they don't have a real thermostat, and depending on the temperature of your house they can underheat or overheat the water. Personally, I would return the heater and buy a 25 watt heater with an adjustable temperature dial--for only a few dollars more you will have the security of a working thermostat that will keep the temperature stable and heat the water exactly to the degree that it is set. It's definitely worth the investment.
If your fish has been exposed to ammonia for awhile and you think that part of the problem might be ammonia poisoning, you should look into getting some methylene blue. It will help prevent secondary infections and recover from the poisoning more quickly.