I have 4 YBS in a alrge tank with 2 filters going

I have to clean the filter sponges every week and a bit as it gets messy inside...if you want to make sure its crystal clean water then get an external filter, try get one thats made for cleaning a larger tank than yours, for example a 150ltr tank will be cleaned better with a filter for a 250ltr tank

Turtles are very messy creatures! My 4 make the filters I have go absolutely disgusting! But thats the joys of having them and I wouldnt change them for the world! They even eat out of my hands...Just the male 1 doesnt like my other half's brother, the only person he tried to bite haha!