There are many possibles for the fry losses. Without knowing all the data, we are only guessing. If you could give us the following, someone should be able to advise.
Tank size, number of fry in it
Water parameters (GH, pH, temperature)
Frequency of water changes and volume
Feeding--what foods, how much
Ammonia, nitrite, nitrate readings
Substances entering the tank water: conditioner (which, how much), any others
Bare tanks, or... ?
Light over the tank?