If they are turning white then they are not fertile. It might be that you don't have a male and female...are you sure about the sexes? If they were eating the eggs I would tell you to remove the slate and put slate and all in a 10 gal tank for them to hatch, but since they left them on the slate this time and they turned white, my guess is that you don't have a male and female, or either the male isn't doing his job.
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Aquatics Unlimited