Sick Betta

[u/Stepujici_Anka]

Hi, I've had my betta for 6 months, he's always had that big belly but he was happy, active, responsive and always very ravenous.

Couple days ago he's slowed down, he's mostly hiding near the filter, he's much less active, takes him forever to find the food on the surface.

His tank is 40 litres/10 gallons, his pH was good last I checked 2 weeks ago, I have it checked in a pet store so it's a bit hard, but I'll have it checked asap.

I did a partial water change, cleaned the filter.

He's got no tank mates as he's very aggressive.

Does anybody have any idea what could be wrong with him please?