Keep a permanent thermometer stuck to the glass or in the water.
This would be a better question for an aquarium and fish forum. Using the tank as a thermal sink sounds like a bad idea for the fish, but I know little about aquaculture.
http://www.fishlore.com/fishforum/
http://www.fishforums.com/forum/
![[Hearth.com] Fish Tank [Hearth.com] Fish Tank](https://www.hearth.com/talk/data/attachments/166/166960-a14616ed185a19e4fe62d81f64e7e743.jpg?hash=CX_cRh0rzS)
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