Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Shift, the usual guideline for flow (especially if you have CO2) is to have 10X the GPH as you have gallons of water.
An FX5 is rated at 600GPH with media, probably optimistic and based on new media with no clogging. Either a Koralia Nano 240 or 425 would be possibilities for your tank.
It depends on your tank setup as well. If you're planning dense plants filling much of the tank, go for the 425. Wide open spaces and low growing plants or carpets, the smaller one might be better to avoid disturbing the fish, especially since they'll have less places to get away from the flow and sleep.
There are also plenty of cheap Koralia clones on Ebay and such. I originally bought one rated 1,321GPH for my 46G, expecting it to significantly underperform its spec. It most certainly did not, and was far too much flow. The build quality seemed surprisingly good. No idea about longevity, as I now use it only for brief periods when doing the "One-Two Punch" treatment. For normal flow I'm using the Koralia Nano 240, and it's perfect for my densely planted but smaller 46G, which also has less flow from the canister. The clones don't seem to be available at this low a GPH.