Marineland Explorer tank runs hot!

I have a 2-gal Explorer tank housing a few White Clouds and Rosies, and 
note that the water temp routinely is in the range of 5 to 7 degrees F 
warmer than the room air temp.  Since the Explorer has NO electric light of 
any kind, and no heater, I wonder where the heat is coming from.  I do see 
that the pump housing (similar to several of the smaller Marineland tanks) 
feels a bit warm.

Anybody have any other explanation or experience with this tank?  It seems 
to be the smallest with a bio-wheel.

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Charles Gillen -- Reston, Virginia, USA