Chillers aren't so much rated by gallons, like heaters are, but rather how many degrees down you want to bring the water...
If you want to bring 200 gallons down 10 degrees, a 1/5 will do that, but that is it's max capacity. If you want to bring it down by 15 degrees then you'd need a 1/4, but again, that's it's max capacity.
If "YOU" wanna bring 200 gallons of water down by 2 or 3 degrees then a 1/5 would be fine.
If your looking to bring it down 8 or 9 degrees, then for less stress on the chiller, you'd want a 1/4.
You cannot go wrong by having a unit that's to big, however it's redundant and needlessly expensive.
I myself have a 1 hp on 150 gallons, and it turns on and off through out the day, Mine came with the system, but if I were to buy my own, I'd prolly only get a 1/4, cause I keep my tank at a steady 83 degrees.
I am not sure what size chiller I should get to make this temp change. I do have exit holes on the canopy and have a fan blowing room air into it as well.
