Yeah the Juwel filter units can be a pain. I had to do maintenance on mine every 2 weeks or it would slow down too much, also it never really did a good job. My powerhead stopped working 3 times the last time it happened it was out of warranty so I brought and external canister.
I just had to cut gap out of the middle shelf to fit in, I made a wee stand to get it a bit higher up so the pump has less head (more flow).
Then I heated the lid with a heat gun and it molded amazingly easily around the filters pipework so the lid still sits flush on top.
Tank looks much better without the internal and the new filter unit only needs maintenance every few months. I am still using the Juwel heater, it is a pretty good heater you just need to get some suction cups for it.
Also before sometimes the fish would get stuck in behind the filter and i would have to blow them out with a drinking straw.