I have used a Sunterra 200850 Pond Pump, 850 GPH with no problems. Honestly, I was worried it would be a really cheap piece of junk, but it has a 2 year warranty; so, I took the risk. I got it for $53.15 with free 2-day shipping. That's hard to beat.

I know a lot of folks recommend you have a pump that's 200% or 300% of the volume of your system per hour, but I just go by the plants/fish and levels in the water. I'm using that for about 3000 gallons. It's not just about the gallons though. Everyone's system is VERY different, I have a low stocking density, and I take advantage of every way possible to create nitrification and oxygenation. I have no data to back this up; so, I don't recommend you try.

So, buy 2 or 3 of those for your system, I guess...