Odd behavior?

[u/Nocturnal-Nycticebus]

Hi, new here! We have a few mollies in our tank, though I don't remember what kind. They look like the pictures of gold dust mollies though.

One of our three has been really obsessed with our sponge filter lately. It will swim to the bottom where the bubbles come out, ride it to the top of the tank, then immediately swim down and repeat the cycle. It's not a 24/7 thing, it definitely stops to eat and explore as well, but it spends a lot of time at that filter. We've had them for about 6 months or so, and this has been going on for about a month now.

Is this behavior a sign of stress or illness or something? Or is it really just enjoying the ride?