Join Date: Jul 2010
Location: Outside Philadelphia, PA
I don't think this would work at all. For a tank that large, your going to need a lot more.
First, do you really need to lower the water temp? 79 F isn't that bad. Also, can you reduce the amount of heat being added to your tank? Every pump in the tank will add some heat to the water. Lighting can also add a lot of heat. If you can cut some of these back, you may not need to do anything else.
You might be able to lower the water temp by using a screen top on the tank and having a small fan blow across the water surface. This causes the water to evaporate more, and coo the tank. It's possible to control the fan with an Inkbird controller. Get one that will handle a chiller and a heater. The down side it that evaporative cooling is limited, and also depends on the humidity in the room. Dry air works lots better.
If that doesn't work, the other alternative is a regular aquarium chiller. However for a 75 gal tank, your going to need something large. That's going to be expensive, about $750 to $1000, and be expensive to run. I'd only do this if there was no other way.