i've heard that boiled egg yolk can be used to feed fry, is this true? if so how do you prepare it and how do you feed it to the fry?

Yes, it can but it's very messy. The product Liquifry by Interpet is a similar option. I've read for the egg yolk, boil the egg, remove the yolk, and squeeze it through cheese cloth. You would add in a little bit of it 2-4 times a day, probably pre-mixed with some tank water to disperse it better. It will foul the water so it's hard to get just the right amount so that they get enough food but don't die from the pollution.