You have a nice simple tank setup, I like that. And a lovely axolotl.:famous: I agree that it's probably male.
Under the axie in the 3rd photo... are those food pellets, or pebbles? If it's food, you are offering too much at once. If it's pebbles, they are a dangerous size for swallowing.
The filter appears to have 2 possible problems: it is a type that generates heat, and it doesn't appear to clean very well. If keeping the temperature low becomes an issue, you might want to consider a bubble-driven filter of some kind.
Thanks for the photos:famous: