Fx5 is much more powerful than ACs. I used to run two AC110s on my 75 and even with the extensions found myself doing a substrate sweep at least once a day
75 gal or smaller 2 AC 110 will work great and they are really easy to maintain plus you have the security of 2 filters incase one breaks. Anything larger than 75 gal a fx5 would be alot better.
thanks for your responses guys! i have 135, running 2 AC 110, ( used to have fx5) but it was just a big pain doing all the cleaning for fx5...however, im thinking about getting back to an fx5 if it will improve my water. currently running with a bunch of community fish: cardinals, rasboras, clown loaches, + discus + plants.
i own 2 ac110s for my 120gallon both with no carbon and 1000grams of biomax each i only need to clean them once a month and nitrates never go over 5ppm. also consider the price. you can get 2 ac110s and a circulation pump for cheaper then 1 fx5. This is coming from a person who also runs 2 fx5s on his 250 gallon. with such small fish i would just stick to ACs.