I would perform a large water change, it's probably just some waste build-up in your system
I saw one the previous post that you're barely changing any water in the weeks, so I'd go for a 75% change now! Take out a good 35 gallons or so, but be careful so that the replacement water will have similar temperature when you fill it back in
It's better to change about 50% of the water a week, that way the nitrates won't be able to build up as fast or easy as they do with those tiny water changes
Think of it as letting in fresh air into a small room!!
I'd recommend some beneficial bacteria, it's always good to add that to the water from time to time to keep your bio-filter going! There are plenty of brands available out there! I use something called "fluid filter media" by EasyLife, but you could look around and see what would suit your needs more. There's usually information on the bottle how to use it and what it does!
If the fish won't get better in a day or two, you may want to test the water unless you did that before the water change. Sometimes pH changes and such can give the fish strange symptoms. If the water is still good, you may want to quarantine the fish and treat it for a bacterial infection. But since all the other fish are acting normal, there's nothing to worry about. The water change will probably do the trick