Filter Clogging with Dirt

Hi everyone, I've recently set up an indoors pond for my 2 female yellowbellies - I have made a large basking/nesting platform for them and they seem happy with it. However after about a week and a half the filter (Fluval FX6) is barely dribbling out and the water is getting very murky.

They don't like nesting boxes and just stress themselves out when put in them, so that's why I decided to go with an inbuilt nesting spot.

When they go into the dirt it sticks to them and they then jump back in, kicking dirt into the water and bringing it with them. What should I do about this? I've seen suggestions of putting a pre-filter on the intake, but I'm not sure if this is wise as they've already eaten sponge filters in the past and something like cheesecloth might clog immediately.