Depends on what your average ambient room temperature is and what the pull down temperature you need for the tank.
Say if your tank temperature is constantly 87 degrees because you do not AC the room and you want to pull down 10 degrees to a tank temperature of 77 , then you might consider a 1/4 hp chiller. Or the room is not as warm or you do have an AC but the tank temperature is constantly staying at 81 degrees and you only need to pull down 3~4 degrees to a tank temperature of 77~78, then you can probably consider a 1/10th hp chiller.
Just remember that any chiller will add heat to the room or area that it is housed.
a 4 inch fan blowing on the water surface cooled my water about 3-5 degrees. 87 to 82/84. lost 2 gallons of water to evaporation a day. The room needs to be well ventilated, otherwise it'll feel like a green house.