I had an FX5 set up for over a year on a 75 gallon bowfront tank. The filter is rather time consuming to clean compared to some others I have used. My 75 is a non-CO2 tank and I find it much easier to just use two AquaClear 70 HOB filters now instead of the FX5, but that might not work as well with CO2. I ran into issues with the FX5, because there is so much filter sponge area in the canister that when I cleaned the filter it seemed to always cause a large disruption to the balance of the tank for some reason. As a result algae would get the upper hand for a few days while everything found equilibrium again. I cleaned the filter religiously every two weeks, so it was not an issue of waiting too long. In reality it seemed like the filter barely had anything in it when I would clean it, but if I didn't clean it the water would start to show symptoms of too much dissolved organics really soon after (surface bubbles in the aquarium would start taking longer to pop). One of the sections of the canister was also completely filled with ceramic biomedia that I didn't touch when cleaning the rest of the filter and that didn't seem to help. This happened even with gentle squeezing of the sponges in old aquarium water, so I don't think I was killing all the bacteria with each cleaning. That issue might have been something I was doing that I didn't realize, and not the issue of the filter itself (or of the filter being mismatched with my application). The filtering capacity was great, but I would say that it was way too much for my tank. Another problem I had was that the filter only comes with two output nozzles that are right next to each other. You can angle them and swivel them different directions, but you can't put one on the other side of the tank or move them apart from each other. That meant that, because of the high flow rate of the filter, I had a very high velocity of water coming out of two distinct points in the aquarium. It was almost impossible for me to get good circulation in the whole tank without there being a large area of the tank volume where the current was too fast for the fish to feel comfortable, or where all the stem plants were just blowing around like crazy and eventually disintegrating in the current. I was able to prevent this problem to some extent by having one of the nozzles firing straight at the back of the tank glass, which was an inch or so from the nozzle. That way the force of the water was instantly diffused in all directions by the back glass and slowed down considerably. I used the filter that way for the last 6 months without any problems at all. A long spray bar might have solved that problem altogether, but to be honest I got tired of the hassle of cleaning the filter long before I wanted to modify it to perform better. I think the FX5 would work much better with 5-6 return nozzles instead of two so the return velocity would be much lower, and also running on a 100 gallon or larger tank that has a large fish load. My 75 had about 14 2-3" long fish of varying species and I think that was too little of a fish load for the filter. Other than that though, the filter worked great, and wasn't noisy at all.
Another thing I forgot until now is that the filter has an electronic timer built into it that shuts off the pump periodically for a few minutes several times a day. This is done in order to let out any air bubbles that may have accumulated in the canister. After a few minutes of being shut off several times a day, the impeller would instantly turn back on and that instant burst of circulation in the tank would scare all of my bala sharks. They would freak out and smack into the glass a few times before settling back down. That was actually my main reason for trying a different setup.