Why is it my Angelfish fry keep disappearing?

My two healthy Angelfish have spawned good, fertile eggs three times! The first time, all the other fish were removed from the aquarium. Parents and babies were kept alone! Babies hatched and went through the regular growth process. On the 7th. day, not one was seen in the tank. The parents gave it a second try about a month later. This time, the parents were also removed...all went fine until the 7th. day, when again, they just "disappeared". I gave up trying to raise Angelfish, when again they surprised me with another large batch of eggs. Great! I was very enthusiastic again! They went through all the stages...hatch-lings, wigglers, and free swimmers! Newly hatched brine shrimp was fed and still, on the 7th. day...you guessed it, they were no more! Filter was also turned OFF. Now, I'm suspecting the aquarium light should not be turned off at night...should I keep the light on 24/7?