Cold water holds more gases than warm water -- so the heater is just heating the saturated cold water up. It should clear up in a few days once the entire tank is uniformly heated. Note that since the water apparantly does contain a saturation of gases you will have to make sure when you do water changes that the replacement water is the same temp as the tank. Otherwise these little bubbles will form on everything -- including the fish and possibly in their gills and since they cannot take oxygen out of air you effectively can suffocate them or given them the equivalent of what deep sea divers call The Bends. Also, dissolved CO2 in the water can artifically lower your pH until the CO2 diffuses out, so you could cause some pH swings in the tank too.
Just as a suggestion, for some time now I have used an old tank that I fill with the water I plan to use for the water change and I run a filter and a heater so that the water is agitated (gets the gases out quicker) and therefore the same Temp and pH as the tank water. I usually just turn everything on the night before I plan on doing a water change.
