A healthy fish doesn't get sucked up against the intake at all, not even for a moment. This is an indication of weakness.... so be prepared to lose this one too. I'm real sorry, AAF.
I don't know the rest of the history yet for this tank, but you asked about removing the uneaten food. Most definitely, you'd want to remove that right away as you know already there are health issues present.
For goldfish in a 55, your total filtration should be at least rated at 250 gallons. I would tend more toward 300 or even higher. Multiple filters are better than a single ones too, so consider having a canister and a hob.
You asked about doing a water change to remove the food... are you actually cleaning the gravel at the same time? (Or sand?.... what kind of substrate is this?