If the patch appeared overnight then it is a bad bacterial infection and needs medicating immediately otherwise the fish will, assuming it hasn't already.
See what the local pet shop has in the way of anti-biotics. Tetracycline, Kanamycin or whatever.
Keep the fish in a separate quarantine tank for treatment because the anti-biotics will wipe out the filter bacteria.
Monitor the remaining fish for similar symptoms. If other fish develop the same symptoms then you will probably have to treat the main tank.
To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge. Do a 75% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
Thanks for the detailed response! It’s a brand new tank that’s been cycled for a little over a week. 0 ammonia, 0 nitrites, and 10-20 nitrates. Can’t tell which one. I’m brand new to the hobby and would be crushed if all fish went to ****. I’ll keep my eye out. Is it possible to know when fish will start showing symptoms when affected?