My Betta fish has started behaving strangely - hanging in the top corners of his tank.?

I've had him for almost 6 months now (along with 1 x goldfish, 1 x Oranda, 1 x Black Moor & 1 Pleccy). No probs. there, they all get on fine. And yes, I know it's not the best combination - despite the guys at the shop where I bought them telling me that Betta's can live fine in cold water! Not impressed with them anymore!! Anyway - my problem is that Schumy (the Betta), has started behaving strangely these last few days - he's hanging at the top of the tank in the corners, and keeps laying on his side. He seems to be struggling to keep upright, and with swimming to the bottom of the tank. He's eating ok, but just seems really lethargic. Is it because the weather's turned cooler, or does he have swim-bladder problems? If it's the water temp. do you think it'd be ok to put a heater in and warm the tank a little (bearing in mind my other fish are definitely cold water guys). I'm going to buy a bigger tank this weekend anyway as they've grown plenty. HELP!! Thanks PS: sorry to go on.