I have an EX20 on my 10g fry tank. It gets to the point where the water doesn't even flow through it anymore. I have to take it off the tank, take apart and clean off the impeller. If you've never done this, that could be your problem. You need to do this at least once a month and periodically check to make sure the water is flowing through it at a decent volume. Also, you need to clean out the area where the filter media cartridges sit. Mulm can build up here and it needs to be removed.
What I do is take out some tank water, or some freshly dechlorinated water, and put all the filter media in it. Then I remove the filter from the tank and put it in my slop sink in the laundry room. Using an old toothbrush, I clean out the inside of the filter with hot water, as hot as my hands can tolerate.
As for the impeller assembly, it comes off of the filter housing, and comes right apart if you work it at the seam. grab the impeller itself and pull it out. At this point you may want to have a drain plug in, in case the little rubber knobs on the ends of the shaft pop off. You don't want to lose them. If they stick in the holes, that's fine, just don't lose them. Take the toothbrush and gently scrub the impeller and magnet under running water. Then I use the toothbrush to get inside the impeller assembly and scrub out anything I can get to (gunk builds up in there) and run it under high-pressure warm/hot water to get all the gunk out. A point that you need to get into is the hole where the magnet goes for the impeller, just stick the toothbrush in at an angle so the bristles get in there, and twist around a few times.
Put it all back together, set on tank, plug in and insert all your media.
FYI on the bio-filters, you can rinse those in tank water every time you do a water change. The bacteria sticks to the surface very well, I just dip a couple times to get any chunks off, which there never are.
I don't use the carbon filters on mine, but you are supposed to replace those every couple weeks or so. I use a bag of Seachem Purigen instead.
I also have a sponge on the intake, for my fry, so mine gets gunkier on the inside, but this level of cleaning in general will ensure good water flow over the bio-media and keep the filter free of gunk, which can cause problems. At least is causes problems in UGF and canister filters. Any filtration system needs a good cleaning every now and then, it depends on the bio-load of the tank, capacity of filter, plant life, frequency of feeding, etc.
I would test the water to make sure you don't have a Nitrate problem (may be due to filter gunk buildup) and then clean the filter no matter what, just so you know how to do it, and can see how gunky it got in 5 months, so you have an idea of how often you need to clean it.
Hope this helps!
