Hi Hels
What brand of fry food is giving you instructions to use it before they are hatched?

I think it might be something that will encourage the growth of microscopic bacteria that will actually be the food for the fry. That's one of the things that will happen with the LiquiFry I use, but the directions don't mention it.
As the eggs mature they will darken somewhat and become more firm. If an egg has fungus it will stand out by it's appearance and you will know it immediately. Don't worry if there are only a few of them since these were probably eggs that are defective or infertile. All batches have a few. In most cases it's fine to leave them alone because the eggs hatch before it can spread and cause harm.
