Possibly sick goldfish?

Hello! Novice here. ​I have a goldfish named tiger that I got at a fair (I know, I know, long story) and he's been acting weirdly. I got him around September-november and he's never had issues, but I noticed food pellets at the bottom of his tank yesterday, and by the bleached color I'm assuming they'd been there at least a day.

I fed him one more time this morning and kept watch for a while. He snatched up a few pellets as they sank, and grabbed one more from the sand and started swimming​ around while (I assume​ sifting through it to filter out the sand). After a while he spat up the sand. And the pellet. And all the other pellets I had just seen him eat. I've decided to stop feeding him for now, mostly for the sake of his water quality (i assume those pellets breaking down will be releasing ammonia. Should I try to remove them?) But I'm not sure what else to do.

We had just done a 40-50% water change a few days ago, but I just did a water test ​this morning ​and nitrates were high, and ​nitrites were near nothing (which is usual in ​my ​tank, I'm not ​sure if ​that's normal). ​I'm considering giving him de-shelled peas​ because I heard those are helpful, but I don't know if he'll be able to eat that, either. My only other idea is to do another water change.

Any ideas on what this is and if there's anything I can do? Thanks.