Play sand is great, but there can be small stones hidden within, so rinse thoroughly, i use a seive and seive the sand first, you will be suprised how many stones are hidden. There are many different substrates, any type of gravel that would fit in your axolotls mouth is a no-no, as gravel and small stones will cause impaction and an unwell axolotl, fine sands, coloured sands or even a bare bottom tank are other options, just check that no chemicals will leach into your water and always give it a thorough rinse before adding.