Hello eereptiles,
From what I've read it seems that if an axolotl is kept on gravel small enough to fit into his/her mouth it is pretty inevitable that it will happen. This can lead to impaction, of course, so should be avoided at all costs.
There are quite a few options though:
Many people keep the bottom of their tanks completely bare. This makes clean-up very easy but some claim that axolotls can get stressed by slipping on the smooth glass. Many of the wise ones on this website do employ this method though.
Another option is to use sand, though you have to make sure that the layer is less than an inch thick or gas bubbles that mess with your water chemistry can form.
A final option is to use large pebbles and rocks such as large river rocks that are much too large to fit into the axolotl's mouth. Try to make sure that there isn't a single rock smaller than the axolotl's head. This can be a bit difficult during cleaning, however, as uneaten food and detritus tend to slip between the gaps in the rocks. With diligence, large rocks can be a very effective and attractive substrate.
As long as the substrate is either too large to fit in your axolotls' mouths or so small (1mm or so) that it will pass straight through your axolotls' systems without a problem anything is fine, and all of the options have advantages and disadvantages. Feel free to make it look as aesthetically pleasing as you'd like, as long as it is safe and you keep it clean!
