I've been looking into canister filters and they list 2 different numbers in gph--pump output and filter circulation. Which one do you consider when you're trying to meet the 10Xs filtration rule?

For example, one filter listed its capacity as:

Pump output--250 gph
Filter circulation--172 gph

So is this an appropriate filter for a 25 gal or 17 gal?

I'm pretty sure that it's the filter circulation number that you look at, but keep in mind that cannister filters are more efficient than other types (hold more media and have little, if any, bypass - in other words all the water flows throught the media, not around it) and the 10X rule doesn't apply. That is why for instance a Filstar xp3 is rated for a 175g tank, while its GPH is only 325. Same with my Eheim 2028 - 120g tank and the filter pumps 227gph and it is way more than adequate!