make sure you keep the in-tank conditions constant for about 2 weeks, which is hard at this time of year, temperature-wise. I would clean the tank about a week before you think she'll lay (or a few days after they breed) so that the water is stable by the time the eggs hatch and you won't desperately need to change it. You may need to test the water's pH/alkalinity/hardness? If they're dropping dead, I would try a different combination of parents, if such a thing allows. you can try giving them lettuce too =] good luck!