Thanks, y'all.
I just got back from a meeting with the builder, architect, and geothermal guy. The builder is planning on completely isolating the fish room from the rest of the house (closed-cell foam). The geothermal guy is planning on running a room zone (heat and cool) into the fish room but essentially we're planning on controlling the temperature in the room with the thermal mass of the water. That is, I'll also have a heating and cooling zone that's the fish tanks and they'll basically set the temperature for the room--the whole room thermostat will be for emergencies or if, for some untold reason that I can't possibly imagine, there's ever not a huge number of fish tanks around.
As for humidity, the fish room will have its own separate air handler so humid air isn't mixed in with the rest of the house. It'll have a dehumidifier. The problem is the heat of the summer and he's off thinking about what to do about that.