First of all, I apologize for the bad pun in the title, but I couldn't resist...
I had two problems in two tanks, as follows:
5 gallon, divided: side with filter clean and clear; side without filtration no water movement and scum forming on surface. Filter: Aqua-tech 5-15
20H: Algae, suspicous fishy smell. Filter: Tetra Whisper EX20.
In conjuction with a partial tank cleaning (removed algae and 25% water changes in both tanks), I decided to double the filteration in both tanks.
First, I retired the Aqua Tech 5-15 (thoughly took apart the filter, cleaned with weak bleach solution, througly rinsed and let air dry). It's now in storage as an emergency back up.
Next, I took the Tetra Whisper EX 20 and moved it to the 5 gallon. I added extra plants to buffer the current. Two days out and no scum formation (which was almost immediate with the old filter), and there is water circulation on the none filter side. Yay!
Then, the 20H got a new Tetra whisper EX45, which doubled the Gph. Again, I buffered the outflow with plants, and everything is fine. Going forward, I'll be interested in seeing how the increased filtration impacts water quality...