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Hi, I'm wondering about sand. I've had it in my Oscar aquarium & various turtle aquariums but have ended up with ammonia & nitrate problems. Even though 'gunk' sits on top of sand, that's fine for regular tropical fish but disruptive creatures like RES's & Oscars is completely different. I believe they mix it all up & you can't syphon sand like gravel unless you want an expensive plumbing bill.

It may be that you need a stronger filter.
When the turtle swishes the gunk around, the filter should clear it up.
I have had better luck with sand, than with a bare bottom tank or rocks.