His stomach looks sunken in a little bit but doesn't seem too bad, I've definitely seen worse. But if you've also seen white stringy poop then there is more than likely a problem... Are the fish that seem to be having the problems all eating? If they're all eating then the flakes will be the way to go, but if some of them aren't eating then you'll probably want to just run to a fish or pet store and grab some medication and just treat your tank since the flakes would be useless to any fish that refuses to eat them. As for what flakes or medication to get, you'll want to wait for someone with more experience dealing with that kind of stuff. There's quite a few different kinds of internal parasites that could be the problem so I don't wanna steer you in the wrong direction. If you don't get any response by early to mid afternoon tomorrow, I'd just bump this post back to the top and ask again. I'm sure someone will be able to help you out though