Could be swim bladder disorder, often acts up in full bodied (round bodied) goldfish. This can be aggrevated by the continual feeding of dry or flaked food because that has air in the food.

Try feeding him nothing but peas for a few days and see if it makes a difference.

Take a few frozen peas, let thaw out, slide the outside round 'skin' off, and feed the centre 'halves' to the fish.

Quick method: take about 6 or 8 peas, place on paper towel, microwave for 20 to 40 seconds until thawed.

40 seconds may be too long... unless you put the peas in a small container with a bit of water (which is my method). You don't want the peas to toughen up (dry out) while defrosting.

Peas have worked wonders for me inre my koi out in the pond (when I carelessly planted something that wound up being toxic to the koi and they were munching on it!)

All fantails swim oddly, the extreem inbreeding has not been beneficial for there quality of life.

We added a bit of salt to the tank, since water tests showed none(and we have never added any so that seemed obvious). I will feed them some peas this weekend. I am keeping a close eye on these fish now!