Again this is a function, usually of too much feed. The extra feed in teh water can irritate the gills and the fish produce a mucus to rid them of the irritants. A sign that your fish have gill disease is that the gill plates or operculum are flared out. As this is likely to happen when you are close to releasing the fish, you can release them sooner or do salt bath as described above. Again the best way to prevent this is to make sure you do not over feed and that your water is clean.
Hatching can occur basically anythime after the eggs ahve eyed.
The causes for premature hatch are not clear, but it seems that once the fish hatch, they contiue to develop as if they were still in the egg shell. There is really not much we can do, but careful handling during pickup, transport and placing the eggs in the tank can help reduce this problem. If they all did, then we would be overrun with them. If you notice one or two fish dead in your tank, not to worry. If you notice many per day then you have a problem.