If they really can't swim to the surface, then there is a swim bladder problem. Try keeping them for a couple of days in the shallow water of a "turtle bowl" or plastic shoe box. Sometimes that helps, though it may be too late.

Could you put a floating plant in the tank, so the fry have shelter and feel secure in the upper part of the water column? Hornwort is good in hard water tanks. I think water sprite is the best of the fry shelters. Najas will do. (Also, the fry will nibble microscopic creatures like rotifers from those plants. They are a useful dietary supplement.)

With plants, you probably do need to leave the light on longer. (See that nuggets from a plant program article.) You also need at least one watt per gallon of water. Two watts/gallon is better.