what size chiller for a 250g.?

I just got a chiller for my 120 which is about 131 Gallons net and I purchased a 1/3hp chiller...

A 1/15hp will have no chance on a 250gal system.
You need to look at the pulldown stats...They are a chart that tells you how many degrees a chiller can pull the temp down per volume of water.

I bought a TECO at the Rockford Fragfest and it handles my system very easily....
I would consider a 1/2hp for your tank.

eins

 

If your on a restricted budget, I saw a guy mod a dorm fridge for his 125. it worked remarkably well. How ever many feet of flex tubing you can shove in and then fill it w/a good conductor. He had his full of that slimey stuff that you grow plants and keep flowers in. I havent the slightest on where to get all the stuff, and how to make it work? But it seemed to work well 4 him

 

I have a 150, and I don't use a chiller, but my lighting is only 3x150w metal halides, and some actinics.   How much heat your lights put out vs the volume of water you have vs the number of degrees per day climb you're comfortable with are the big variables.   My tank is set to a min of 79, and will climb as high as 82.   I keep the room it's in cooled to 73 or below degrees with a window unit air conditioner.
If his heat problem is small alot of times a fan clipped onto the sump will easily keep it under control....  A great deal of heat energy is lost when water evaporates.
David