I had a huge, ugly, very expensive filter with UV lamp. It has 4 large sponge blocks which were suspended below a plate..Theoretically the water had to pass up through these sponges over the top of the plate and out. It didn't work, and if the sponges were blocked, which always happened when I was out, it either shut off the pump through back pressure and backflushed all the nasties back into the pond, or it overflowed, half emptying the pond. The "biological" part never worked.
After a year I gave up with it. I bought an Ecocell which came minus UV, and only 1 filter sponge over the biological media. I got a pack of 3 sponges of different density, and 3 weeks later the pond was much cleaner. I had to clean the sponges every few days at first while it got to grips with the pea soup. Now I can see almost to the bottom, and the water is coming out of the filter almost clear, if still tinged fainly green. Once a month I rinse the sponges in a bucket of pond water. If they are too bad they get shoved through the washing machine on a cold rinse programme. Works like a charm. There are enough biological entities left in the filter base so it doesn't seem to matter.