I have noticed that the temperature of my reef tank is steadily raising the last few weeks. I have had this heater in the tank for just over a year and for most of that time the digital temp. reading was 79.6F. Over the last few weeks the reading has increased to 82.2F with the house temperature at 76F.
There is some growth on the heater so as of right now the heater is in a bucket of r/o and vinegar.
My questions are; is the increased temp. due to the growth? Is this the sign of a heater malfunction?

I've had heaters in freshwater tanks for many years without a problem is the normal off the shelf heater adequate for a saltwater tank? This is a submersible 200 watt heater made in China. I forget the brand. Tank size is 55 gallon with a 20 gallon sump. The heater was in the main tank.