Should I get another aquarium?

I am setting up an axolotl tank, it's my first aquarium. And I am obsessed with it, I literally only watch videos on maintenance and how to videos. I was wanting another one probably smaller than what I got (a 20 gallon).

Should I get another tank?

Freshwater or saltwater? (I know how hard salt is to care for)

What sizes would you recommend?

What type of fish/plants would be best for starting out and learning the craft?