I'll be the first one to tell ya I'm FAR from an expert, but since you've only had the tank 2-3 days, and you noticed the flashing more after the water change, I'd say it most likely could be just simple change in water parameters. I've noticed when I first started back into the hobby last year that it took me awhile to get the hang of how much water to change, matching the temps, adding the right amount of conditioner etc etc, and when I was off a bit with some of those the fish would flash for a bit after but it didn't last very long after the water change, they would settle down.
As Marty said the sunken bellies could just be a bit of a lack of food since you don't know how the previous owner would feed them, and in the pics of the tank you posted I noticed there was a wide variety of size fish so there may be some that are a bit of food hogs and getting the food before others have a chance. So I'd just watch to see if the flashing continues, make sure they are not showing any signs of ich or white poop as Jenna said, and also feed them on both sides of of the tank (spread it across the tank so all the fish have a chance to get some) and see how it goes. I wouldn't worry too much unless you started seeing some for sure signs of problems like white poop, constant flashing, white spots on the fish etc etc.