My goldfish can't swim straight

So yesterday he was totally fine but today when I got up to change their water I noticed he was floating from side to side underwater or laying on the bottom of the tank and normally when feeding him he rushes and eats it like I don't feed him daily but today he couldn't reach the pellets and just drifted away, I changed the water and placed him on a separate and smaller tank to check up on him but I don't know what else to do, does anyone have any advice?

I'm still pretty new with fish even more with goldfish but I've had this guy for a year now and keep him in a 13 gallon tank alone with a filter and some water plants and feed him floating pellets.