It's 40 gallon breeder tank so it has about 35 gallons on water in there.
One turtle has about 6 inch shell from the neck to his tail. The other one is about half a inch bigger but she is much wider.
Her shell is much much thicker than the boy ( I think his a boy).
The filter has 3 baskets top is two carbon bags, middle basket has two sets of white ceramic rings and the last basket has two black foams. On the side of those baskets it has four long sponges.
For water changes we vacuum all the water out and put fresh water which is treated with the drops (once a month) with one or two partials in between. The filter would be rinsed out including the filter media every other month.
Recently since we started noticing the water get cloudy and smelly, so we began to clean the tubes which were filled with gunk and replaced the carbon. And we are doing full water changes every week, though they need it about every 3 days, in addition to the weekly filter rinse.
Two days later the water was cloudy and smelly again.

I don't know what else to do. They get feed once a day- pellets.
I did notice this happen when I began to feed them red leaf lettuce but I've rinse the filter plenty of times after so I'm sure there is no lettuce residue in there. My mom would drop the lettuce in there and of course they'd wouldn't eat it all and the filter sucked it in.
Thanks for your response Steve!