Boiled egg yolk (not the white) is used in most liquid fry foods sold at pet shops. It can be used as an emergency food for newly hatched fry, and is suitable for tiny fry like Labyrinth fry.
A small amount of boiled egg yolk is fine for fish and contains a lot of protein. If fed to them once or twice a week (in addition to other foods) it will not harm them at all. Fish naturally eat fish eggs in the wild and boiled egg yolk has a similar nutritional value to fish eggs.
The only drawback to boiled egg yolk is uneaten egg yolk will cause the water to go cloudy and can cause ammonia and or nitrite spikes. But if you offer a little bit at a time and remove any uneaten food, it is fine to feed fish, (and birds and most other animals for that matter).
All fish should have a varied diet and get frozen or live foods every day if possible, in addition to dry flake or pellet foods. Variety is the spice of life and variation in a fish's diet will provide them with the most nutrition and best health.
