Have you seen his poop?
Are you familiar with what it would look like if he has internal parasites?
Can you look at his body, gills, eyes mouth, anal vent for any worms or tiny specs white, gray or black, could be fish lice.
As for feeding, a variety of foods would be better than a single food type, could be some deficiency.
My first guess would be internal parasite. The flashing would possibly indicate a Nitrate spike in the water before, or external parasite.
Were they live blood worms? Often live worms cultures can carry unwanted organisms.
This might help with some direction for illness.
Do you have other fish in the tank which could be intimidating this little guy?
edit - adding too that some hiding places would be nice.