Add more flow. You want a minimum of 20x turnover. 40x is better. In my 33 gal I've got about 30x and will add another koralia 2 to bring it up a bit over 40. That should help with your red slime and also keep crap in suspension so it can be removed instead of settling on the sand.
For keeping the sand clean try a couple of big nassarius snails, a strawberry conch, or a tiger tail sea cucumber. They do a good job on mine.
HTH
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