I own the 1/10 HP JBJ and use it to cool down my 30 gallon cube that I keep completely closed with lids. The tank has hardly any evaporation because it's boxed in and that traps heat with the pumps that are inside, I have return pump, UV pump and chiller pump, all SICCE.
I keep the temp at 78.0 F and it takes about 15-17 mins to turn on and cool it down. Because the tank is 3/4 inch cast acrylic it takes almost 2 hours before it turns back on to cool.
JBJ uses an input data scale to enter all values; pumps, heaters, lights just about all you can think about to determine which size to purchase to achieve your goals.
www.jbjaquarium.com
I also own thermoelectric chillers for my two 12 gallon cubes that are not compressor based, much quieter to run and they are more cost effective for my application, however they don't have the power to cool if ambient temps in the room go crazy, such as broken central air here in Florida, something also to think about.
Compressor based chillers are loud, compared to thermoelectric, but much more powerful, and JBJ claims to have one of the quiet compressors on the market.
I have owned the 1/4 HP also for my 90 gallon bowfront I had, worked like a charm.
Also owned Oceanic Chiller that was for a 24 gallon I had in the past, also very nice.
If you have any other questions, I would love to try to assist.