You need to think about the chemical effects of increasing the temperature. Fish produce ammonia (NH3-) as part of their metabolism, very much like ammonia found in human urine as a result of cell respiration. As you may know, ammonia is very toxic to fish, even in tiny concentrations. The concentration of ammonia does of course increase if it was in a smaller volume of water. The bacteria in your filter break down the ammonia into nitrite (NO2), which is equally as dangerous because it binds to the haemoglobin in red blood cells and reduces their ability to transport oxygen, giving rise to a condition called 'brown blood disease'. The toxicity of ammonia and nitrite increases with pH and temperature. Increasing the temperature by two degrees would increase the toxicity of these substances ten-fold, and it's not worth increasing the temperature by just two degrees as it won't make much of a difference to the overall well-being of the fish.