How do you feed them egg yolk? I've heard of a lot of people doing it, but I have no idea how.
I hardboil eggs, and take the yolk out of the white. Then I take a piece, about the size of a pea, and put it in IAL (you can put it in water but, I just like that IAL is anti fungal). Then I crush it in between my fingers, put it into a small bottle (I use a very well washed pill container), put the lid on and shake it until it's all dissolved. Then, when you open it, you can see all the yolk dissolved into floating specks. If there are any big ones left, I use a spoon or fork to break them up, shake again, until it's all dissolved.
You take a medicine dropper--I got a pack of two at the pharmacy--and I use one with an upward curve on it, so I can get under the water, when they are all swimming on top. I suck up a small amount and shoot it in, very slowly, to all the areas the fry are. Just be careful as, it spreads very quickly and, it does end up falling onto the bottom of the fry tank and can go bad quickly. So, just use enough to feed the areas the fry are and that's it. With the left over, I put it into the fridge for the next feeding.
Hope that helps--and, it does work to help the fry grow faster. It is loaded with protein. I wean down to once a day, when I introduce MW or WW/BW and then, stop using it and just go to the worms and, eventually BBS.