Japanese Goldfish experts...please help!?

As with almost all goldie illnesses the very best thing you can do for them is to ensure pristine water conditions and a gentle feeding regime. It sounds simple but these can be resilient little fish whose own immune system can handle most things if given a chance.

Get that melafix out of the tank. Get them salts out. Lots of lovely fresh water changes with dechlorinated water. How many water changes depends on the size of your tank but as long as it's of a decent size and has good filtration I would suggest 25% daily & take the opportunity to vac your gravel to ensure it's also super clean (bacteria can lurk there for a sickly fish).

By the right sized tank I mean something around 60l per goldfish and at least 120l to start with (i.e. 120l for 1, 120l for 2, 180l for 3 and so on).

By good filtration I mean a cycled filter that is rated at 10x the volume of the tank (i.e. for a 120l tank the filter would need to be rated at 1200l per hour).

Get a water testing kit (API Master Kit is a good one) and test your water daily to check it stays at 0ppm ammonia, 0ppm nitrate and 20ppm or less nitrate. If you don't know what a "cycled filter" is or why you are testing your water then research the nitrogen cycle on fish tanks. It's imperative you understand how it works and what you need to do to promote it. Without this the chances of you keeping a healthy fish for very long are very much diminished.

Feeding

Goldies do best fed little and often on a wide variety of foods (not just flakes or pellets). A heavy focus on veggies will keep them healthy - blanched broccoli, cabbage, lettuce, cucumber, courgette or live plants such as elodea for them to munch on. Pellets can then be used to supplement this diet at a pinch every other day. Put your pellets in and count to 30secs. if there are pellets left in the tank at all then cut down the next pinch. Keep doing that until you are feeding only what the fish can eat in 30secs.

Scales can and do grow back on goldies in almost all circumstances (though sometimes they grow back transparent or a different colour) so just focus on making his tank as healthy as possible and he'll focus on getting himself better.