Axies don't have swim bladders. And pet shop assistants generally don't know anything about axolotls - their job is to get you to spend some money.
DO NOT add chemicals of any kind to an axie tank unless it is on the advice of a vet.
Axies float because of 3 things:
1. they want to, it's voluntary, and they like it.
2. they need to poop, usually their bum floats. In small juveniles they can also get air bubbled in the digestice tract. It passes when they poop.
3. they have a genetic condition that means they can breathe in air but can't breathe out again, causing stressful floating.
What are you feeding your axie?
Can your axie get down to the floor of it's tank when it wants to?
Does it appear upset about the floating?