It's a awesome build thread. You mention you built fish room within area with a register but do you have a return? Without a return to remove old air it will not cool room
I learned this from my AC guy when my brand new house had hot bedrooms in summer (builder put only one return for entire second floor)
One end of the fish room is open to the utility room, that is where the main return for the basement is. The air is moving and getting exchanged on a regular basis.