If you're really concerned though, you can name it with a name tag, as this prevents any mob from despawning, and it'll also serve as a nice way to mark your special new companion.
Unfortunately, she's not strong in the sciences, so she fills her days with researching and writing about all kinds of animals and spends time playing with her adorable but terribly naughty tabby cat, Bell a.
Along with a good filter, your Axolotl will also rely on you to do weekly water changes for them.