Help with betta!

So recently my betta fish has been staying at the bottom of the tank and not being able to swim to the top for long without just letting himself float back down, it’s causing him to have trouble getting up there to eat. He’s in a 9 gallon, heated and filtered tank and his only tank mates are snails that managed to sneak in from aquarium shop plants. What do I do? I’m buying some more water tester today.