My goldfish are swimming around and opening their mouth really wide. It looks like they're gasping?

Okay, so i bought 2 black moor Goldfish and an orange gold fish but i don't know what kind it is, it has black spots on it. Well anyways. The 2 black moor goldfish seem normal until they're just swimming along and there mouth opens really wide, like they're gasping for air, and sometimes they'll stop swimming with their fins and lay sideways and then go back to swimming like nothing happened but when they swim they swim really fast and move around the fish tank like they're on speed or something. And i'm really worried about them, the water is well water, i filled the tank up under the faucet at my house, and there's little pink gravel i put in there, and i have a filter too. Their fish food is Tetra finn goldfish flakes, and i feed them once a day. I don't understand whats wrong with them... HELP!!!!