I also have a marineland stealth heater that I did have in my 35 gal
, I know use it to heat mixed saltwater when doing a water change. It is completly submersible also.. I think if you look around at the LFS
or online you could find 1 or 2 heaters that would be small enough to fit in your sump.. Or you can keep the heater you have and install it in the display tank and just hide it with some rock or something so it is less visible.