Yeah I guess you are right. Here we have the high humidity but usually around 70F. I had a problem with my ventillation system because it worked well for temp. and humidity but when it rains the humidty goes to 100% and the fans cycle on and off because the replacement air is too humid. I had to shut of the humidistat.
Another thing you might consider is a small heat pump with dehumidifier. That would be much less expensive to run than a chiller I would think. Since you have a large tank, temp. fluctuations will be fairly slow so you could dial in the heat pump to keep the room at the temp. needed to maintain the tank.
I suppose a chiller would be simpler. Have you tested the temp. of the soil outside the house?
Check out this
TOTM. Tuan uses an underground loop to chill his large system with excellent results. If I have problems in the Summer i will install the same thing.
