I have 2 penguin 330 bio wheel HOB filters in my tank. Today when i changed the water (its been 3 weeks since my last water change) i noticed that my filter inserts were mushy and if i tried to dunk them in the old tank water or brush them off a bit with my hands, they were falling apart and almost like melting....

this is the first i've noticed this problem in the year of owning this tank setup. I usually change 2 of the 4 filters each month or two depending on what they look like. the only thing i can think of is that i added some custom made baskets for some extra carbon.... Is it possible that the carbon is causing this? My fish have had no problems and in fact seem happier then they were previously.

I replaced 2 of the filters with new ones, but i only had two laying around, so i need to go get more to replace the other 2. I also took the extra carbon out...

What do you all think the problem could be? Or does this sometimes just happen?