Firs, allow me to welcome you to Tropical Fish Keeping forum.
"Flashing" is the term for what you have described, and it can occur as the result of several things. This is why it is very important to carefully observe the fish in the aquarium, looking for other signs as well as flashing.
Ammonia and nitrite poisoning can also cause flashing, but we can probably eliminate these (even though you haven't mentioned test numbers...?) as this seems to be a tank that has been running for a while.
Getting food caught in the gills is a possibility, but this is usually a one-time thing and not likely the case here.
Which leaves some sort of parasite. Ich is one, and this always fist shows up in the gills where we can't seed it, but the flashing is the observable symptom. And as ich is always present in freshwater, it only takes stress on the fish to bring it to the fore.
Gill flukes is another parasitic cause, but if this is the issue, you will probably see the gill area very red, with the operculum (gill cover) extended, and faster respiration than normal.
Assuming this may be ich, if the only thing you have seen is flashing in one or two fish, I would tend to leave things alone. Fish free of stress will be in better health, and thus better able to fight this off.
And that brings me to some other questions. How often are water changes done, and how much volume? Do you do a good cleaning of the substrate at each water change? Having so many substrate fish, this is very important.
Have you tested for ammonia, nitrite and nitrate? If yes, what are the numbers?
The pH you say is 7; is this just natural, or are you doing anything to "balance" the pH? What is the tap water pH for comparison [remember to shake tap water briskly for a few minutes before testing pH to outgass the CO2]?