With regards to the gravel , if it is smaller than the size of the axolotl's head it could eat it . When they consume food they gulp it in, if its a worm for example it may take more than one gulp, and in the process the stones are consumed. Some may pass right through or be regurgitated , and some in time become stuck in side your little axolotl. When I received my axolotl they would produce every now and then pieces of very bright blue gravel, the previous owner told me they were only kept on bare glass for the 4 months that she had them ..previously the pet shop had them in the tank with the gravel. .. I still now get alian gravel small stones appear!