100 watts is a lot, but I've used a 25 watt in just under a gallon and been fine. Watch your thermometer VERY carefully, and you may want to turn the heater down to several degrees below where you want it. His water should be somewhere between 78-80 degrees.
You can get a nice simple glass aquarium thermometer for about $2 at Wal-Mart, or about $4 at Petco or Petsmart.
Grey belly and chin is a sign of severe stress. It can signal the onset of dropsy. Dropsy is a symptom and not a disease. My guess with a 5 gallon tank and 25% water changes once a week is poor water quality. If you've heard of "New Tank Syndrome", this could be exactly what you have. Warm him up (slowly) and give him a 100% water change.
I notice you have white gravel in your tank. Some folks think that white gravel causes additional stress in fish. Other people don't have problems with it. Let's work on temp and water quality first, though.