May be flukes. Best way to confirm is freshwater dip. If after a couple of minutes you see what looks like sesame seeds that fell off- Flukes. If not...... freshwater dip still beneficial
You could do 2 things. Take every fish out and treat. (Probably not needed)
Or you keep them fat and happy. Mayb add some garlic to their food to help them with their slime coat. Do not stress them. Use a uv. And after each round (when te ich goes away) take away the top layer of your sand with a hose.
Had ich. Added fish from a tank with ich. (And showing a spot or 2). New clean fish dont get ich in my tank. And "healthy strong" new fish clear up in a week.