i am assuming that there is access to the space above your tank but i don't see it in any of your pics. are you planning on drywall (the bathroom, green waxed, kind) on the inside of that space or some other type of sheathing?
have you considered a sky light for both lighting and ventilation directly above your tank? i have fans above my tank but i'd rather vent the humidity out of the house if it were possible. unfortuately in my situation it is not possible.
thanks for the updates. your house looks great!!!