I tend to agree with @lksdrinker
: if something substantial appears, it might be a good idea to treat just in case. First, be sure water params, temp and cleaning methods are good so fish are happy. Ich shouldn't go away like that, but there are other parasitic diseases that may go underground after a brief surface appearance, especially when they appear in the gill area.
First, a disclaimer: I haven't had a disease in about a decade, which I attribute to a high-end UVS, plus other typical good things, so my methods are at least ten-years old.
I found PraziPro to be the best overall parasitic worm killer. Seachems ParaGuard is also an excellent ich/parasite killer. I dose PraziPro annually as a 'worming' function. I don't follow the dosing recommendation. I bring a hammer down on the life cycles of parasites. Here is what I do, if you want to try it:
Day 1: Remove carbon/Purigen and filter pads. Clean sponge for maximum water flow. Turn UV
sterilizer off. Do a 50% water change by vacuuming of substrate surface, then add PraziPro.
Day 2 to 4 (3 days): Do nothing
Day 5: 50% water change by vacuuming of substrate surface then add another dose of PraziPro.
Day 6 to 8 (3 days): Do nothing
Day 9: 50% water change by vacuuming of substrate surface then add another dose of PraziPro.
Day 10 to 12 (3 days): Do nothing
Day 13: 50% water change, add carbon/Purigen, filter media and replace sponge filter.
Sterilize equipment in a 1:4 bleach:water mixture.