Pool filter sand? Never heard of that, honestly. As for walnut shells, its all a matter of opinion. ANY substrate can be harmful if you use it wrong and for the wrong reptile. I had my uro on zillas ground walnut shells for about a year before I started using alfalfa pellets. I never had any problems. It was warming and easy to clean for me. I don't pass judgement. It's like if I say that pool filter sand causes impaction. I don't. So I won't comment on it.