I would stay on the once a week changes. Check your numbers and see if anything is high, this may narrow down the culprit.
My first initial guess is that your filter is really dirts, filled with buildup and its being pushed into the tank.
Your gravel may also need a brief cleaning with a vacuum tube.. I usually never need to clean the gravel in a tank, but really old ones tend to get some serious buildup.
Also make sure you aren't over feeding or overstocked on fish.
My only other thought is if you have plecos, lots of them, they tend to be really really messy. I breed ABN Plecos and they make such a mess after eating...