sand sucks

I had a leopard gecko and I realized he was eating his sand. I thougt it was healthy because petco's care sheet said leaperd geckos eat it to help digest their food. I know its bad now because now I know about my leopard gecko, savana monitor and bearded dragon -not to let them eat their sand any more so now I just basically build their hole tank with silicon and liquid nails and poly styrofoam so they can't eat their substrate which I dont use anymore.