What is the best "Tropical Fish Room" insulation?
Been there , done that. My room is a 13' X 13' corner of the garage . Mild climate ( Houston TX) I have 4" fiberglass in the walls, 2 walls of double glass windows , 8" of loose glass plus 4"  of foam in the ceiling . Summer is fine with a small room AC unit. But in winter there is condensation from the Al window frames and glass. The paneled walls are damp and get moldy, the ceiling (drywall) has black mold. In spring I seal the aquariums and wipe everything down with bleach rags. I have 125,75,39 and  20 gals with about 80% glass tops. I would recommend concentration on your interior surface finishes , understanding they will tend to get mold. I have no heat in the room other than the aquarium heaters ; I believe an electric space heater would help by reducing the relative humidity . We enter the garage through the room so there is some air exchange. When I built it ( 14 years ago) the EPA required the mold inhibitors to be removed from caulk, so the caulk around all the windows is black and not cleanable. I understand new inhibitors have been developed  , be sure to get "good" caulk.