We have had very good experiences with the Arctica chillers from Transworld Aquatics. If you're tank is well lit but also well ventilated, you'd probably be looking at around a 1/4 horse although they provide a great calculation where you put in all your pumps, lights, tank size etc and it spits out what size chiller will work best. The nice thing I like about the quarter is that it has a similar footprint to the 1/5th horse, it's just taller. I think it's around 9"x18" or so. We've used several chiller companies in the past, and nothing has performed as consistently as the Arcticas.