How old do the fry need to be before its safe to raise the water level above 6 inches?

Just when they are free swimming, or do I need to wait longer?

As soon as all the fry are swimming horizontally, feeding well and moving up and down in the tank's water, you can raise the water level. I would not recommend raising it all at once. I usually start scooping out water for changes about that time and each time I take out some, I put a little more back in. That way, the water depth increases slowly, day to day.

The shallow water depth is MAINLY helpful in making sure that each newborn fry has the best chance to make it to the surface to grab that all important first gulp of air to fill it's air bladder. Without that, they become sinker - bottom sliders - and do not make it.

If you do not have a gentle bubbler running to break up any scum that may be on the surface - or, more ideally, a sponge filter, this is the best time to introduce it - before the water is really deep......

Thank you Daryl. I have an in tank filter that is air powered. It was the closest thing I could find to a sponge filter. I did replace all the filter media with plain filter floss, so there is not much room for the fry to go exploring in there. After the fry were free swimming I turned up the volume of bubbles a bit since the slight agitation at the waters surface did not seem to bother any of them.