Oh delightful! I won't even tell you what the water looked like after I cleaned the filter when my goldies were sharing with my platies. I will say when I opened the filter I thought the goldies had their own version of the midas touch and that they had magically turned the spongey stuff into a solid block of poop!
Should have seen the giant canister filter at work on the goldfish/pleco tank. A couple months ago I found out that no one had cleaned it. Ever. It had been set up for several years. The way I found out is that it finally clogged and started spewing out the entire tank all over the floor and I had to take it apart. It was like... Black cottage cheese.