Which type of cat litter, I know some do go like that but the one particular tesco one doesn't apparently..There's also moler clay which is supposed to be fine too
I never really had a problem with it mixing, but if I decided to move one of my swords, that did cause it to be pulled up through the sand and turn the water a cloudy red for a bit, I just topped up the sand where the Plant was to cover the mess ;0)