How often do they "Flash"? My fish will Flash after a large WC because my PH drops over time in my tank and when I fill it with fresh water from the tap the ph is a bit higher and I will sometimes see them flash then but its only a few times and they stop and don't do it again. Could something have gotten in the water? chemicals and other stuff can irritate the fish fish. To me if you see them flash everyday it Sounds like Parasites.
There are lots of parasties that can cause this not all are easy to see of know what it is. Ich can stay out of site and only be in the gills at first but if left untreated and if the fish gets stressed it will sped and be seen on the body. Velvet is a parasite that is like ICH but can only be seen if you look very close with a Flash light. It will look like the fish has gold dust on them. Flukes can be on the fish and never seen. What is the fish Breathing like Fast, slow, or normal? any extra slime on the fish?
When was the last time you added a new fish to the tank or to another tank? Because these parasite can be brought from one tank to another pretty easy by just a drop of water. Even New plants can bring in parasites.