What could be wrong with my goldfish?

I have 3 goldfish and yesterday I started noticing one of them was floating upside down on the bottom of the tank. He was still moving his gills and his mouth and swimming around from time to time but today’s he’s not really moving and he’s laying on the bottom again. He looks like he’s dead so I go to take the net... show more I have 3 goldfish and yesterday I started noticing one of them was floating upside down on the bottom of the tank. He was still moving his gills and his mouth and swimming around from time to time but today’s he’s not really moving and he’s laying on the bottom again. He looks like he’s dead so I go to take the net and get him out but when I get the net near him he starts to move his gills. My other fish are perfectly fine and I haven’t done anything new to the tank recently. What should I do? I know this is a silly question but I got these fish for my toddler almost 2 years ago and I’ve gotten attached to the little fellas. I also don’t want the other fish to eat him.
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