A while ago I took my axolotl to the vet and told them I kept her on a fine sand substrate because that's what I always heard you should do. They told me sand causes impactions and instead I should keep them on rocks bigger than their heads. I had always thought if your sand was fine enough, the axolotls could digest it without getting an impaction? I worry their little feet would get crushed in between the rocks. However, I did take that axolotl off the sand and kept her in a barebottomed tank ever since and she hasn't gotten one air bubble. I'm not sure if it's related, but the one with sand get air bubbles every week and spends more time floating then on the bottom. I feed him sinking pellets, so I'm not sure if there's a connection between sand and air bubbles. Do you think it's safe to keep him on sand?