Splish Splash little tank filter question

[spankhurst]

Hi, we have a teeny 21l tank and use the filter that came with it. We lost a fish a month ago, he had a white patch on his side the day before. Now one of the white mountain minnows has cotton wool disease, which I've treated with stuff from the pet shop. The filter looks quite gungy; we change the carbon thing fairly often, but my question is, should we also change the metallic sponge part which is gungy as hell? I've Googled but am none the wiser. DH thinks the gunge is good bacteria but it looks horrid. I do clean the tank and do partial water changes regularly. The tank smells a bit at the moment too, which worries me.

Sponges usually need to be changed every 4 weeks, what fish do you have?

No NEVER change filter sponge unless it's falling apart as this is where the benificial bacteria lives. When you do a water change squeeze it out into the bucket of old tank water. Never rinse uder the tap either. I have a 25L tank with a single Betta and just do that weekly with the water change.

That tank is too small for any fish I'm afraid.

get a much bigger tank fish should be with other fish not on their own! OP think how they live in the wild in shoals