heater or not

I have been running my 36" tank for 10 months now and still do not have a
heater present. the temp never varies much more than 1 or 2 degrees C.
is there something wrong in my system. should it be cooler than this. the
ambient temp is around 15-23C but my temp will drop to 24.6C and through the
day rise to 26.7 in the evening. I think a cooler would be more appropriate.
is it my powerheads or return pump or is the MH light heating the water temp
too much.

Robin