Axolotls will literally “vacuum up” anything that can fit in their mouths! Our breeders aren't named after vacuums without good reason, lol!
As a general rule of thumb, the substrate should be at least twice the size of your lolt's head. Gravel, marbles, pebbles, etc. are big NO NO! These items can impact your lotl’s intestine which may lead to death or an expensive surgical procedure to remove.
Substrate isn’t entirely necessary. Many owners prefer a bare bottom tank because it's easier to spot to clean.
This is totally acceptable.
Sometimes owners use ceramic floor tiles cut to size for the inside tank bottom. This is a great idea--especially if the tiles have some texture to them for the axolotl to grab onto. If your tank bottom is clear, you may consider lining the outside bottom with some type of contact paper to make it unique and custom to you and your lotl. I would suggest to not put it in the tank itself.
At Lizzy's, when the axolotl grows to be about four inches in length, we prefer to use fine aquarium sand. Some owners feel that substrate is better than bare bottom so the axolotl has something to grab onto. This may result in less stress on the animal. Our favorite brand is "Super Naturals" premium aquarium substrate by CaribSea which comes in many colors.
This sand is perfect because it is extremely fine and has nothing artificial in the ingredients. Waste remains on the sands surface, so its extremely easy to spot clean with a turkey baster. We think it looks very natural and it keeps decor and plants more stable making your aquarium aesthetically pleasing. But that's just our two cents.
If you decide to use sand, you'll want to make sure to stir it routinely to prevent harmful bacteria from building up in any air pockets.
Large river rocks are a great addition because they look natural and our breeders like to lay on them. We did try it as the whole substrate once. And, although it is possible to use, we wouldn't recommend it because its hard to spot clean. Waste and debris can get stuck in the crevices causing ammonia spikes. You also run the rare risk they shift causing injury.