Shiz nailed it ,as usual!
The only suggestion I will make is to be aware of the probe on them. Some of them have unplated copper probes. Copper is toxic to caudates. You can determine this by sight- if it is shiny like chrome on a car you are good to go, if it looks like a penny, you should encapsulate it in epoxy or the like.
Also make sure the probe is set and affixed in a spot where your axolotl is unlikely to try and eat it. Some times they will mistake them as a snack.