i feed them flake food daily and the odd treat once or twice a week (blood worm and brine shrimp).
the fish are totaly healthy, it just happens when the light goes off. the tank is good too, except some algae! would the lack of light affect their eye sight?
More variety of frozen foods daphnia very good for digesting the fish food.
Feed the fish some shelled peas to flush them through.
Cook peas for a few minutes, let cool down, pop out of shell, mush between fingers and add to the tank.