I have a Juwel RIO 300,which I think is a good tank,however I use a external filter to back up the built in filter.As for changing the filter foams you only need to change the top layer of filter wool weekly,and change the carbon (if you use it) evrey 4-5 weeks.
I have never replaced any sponges in ...

Have you checked your oxygen levels?,L numbers tend to come from moving waters and therefore require high levels of O2

I keep 7 discus in a 70g with a selection of L-numbers,with no problems.I even have strong water flow and the discus don't mind.I also keep a selection of cory's,2 clown loach and a shoal of cardinals.The largest L No I have is a L137 "bruno",this is aroun 8" and causes no problems at all.
All I ...