[Stoll]

If you're going with the Solaris LED system for lights you shouldn't have any troubles with heat to begin with unless there is an uninsulated heat duct in that wall.

I would set your fan to activate at a certain humidity level because you definitely don't want excess humidity condensing inside your wall. Wood absorbs moisture, and if the wall you're going to build in is connected to a bunch of other interior walls, you could be adding a fair bit of moisture into them.

You could run an air barrier around the inside of the aquarium wall space which would contain the humidity, but you would still need to exhaust it somehow or it will end up condensing on the air barrier and running down onto your floor.