My betta is acting strangely! He changed color and he is kind of listless. He still swims but he is swimming very slowly. He was just fine the day before so i'm kind of stumped! I don't know what to do! Plz respond as soon as possible!

change 25-50% of his water, condition it. and add some aquarium salt (it acts as an antiseptic). if he doesnt improve after 24 hours let me know.

what type of living arrangements is he in?
and
how often do you change the water?

He is in a double tank because i have another male. I change the water at least once every 2 weeks.

Im not sure what a "double" tank is. how many gallons is it is what im asking. And i am also asking what type of tank, like a filtered tank, or a vase.

and when you change the water, do you change all of it, or just part of it? and if its part of it, how much? 20% 50%???