This is quite common Lizzy, the fact of the matter is that there is several thoughts as to why this happens. I had it happen several times and i really dont have a clear answer. Infact, to be completely truthful, i dont think anyone does. You can guess and speculate, but it is still one of those great unsolved mysteries, that seems to have a different answer from book to book that you read on the subject. Certain cichlid species do have to have almost constant food when young..which would explain the rapid growth in these species...but i would be hesitant to say that is the answer, as often the young die almost immediately. Stress, yet another possible factor....but? Heart attacks? Im not sold on that as an answer either. A change of water is quite possible as the mothers secretions may provide a certain amount of protection to them. Even a very slight bacterial attack of a young fish can be fatal, as the slim coats on theyre body when born is very thin. Personally, if you want my guess, i would have to say that from what ive learned about egg bearing species is probably the same cause as the deaths of now free swimming fry. That is, the parents of suck species are constantly grooming parasites, and further more...constantly fanning the young. If you remove say a leaf or rock in a tank with eggs...they will always almost 100% die if there is not water movement provided by like an air stone. Now if you consider how fast eggs are attacked with mold, think of it in terms of fairly young fish. They really have no protection, now say if you could provide them in an isolated area with constand oxygen running over them, proventing attacking of the skin, and mucus, i personally think you will see a higher brood success rate. Most breeders use completely bare tanks with lots of oxygen movement. And most of the fish will breed in a rainy season where oxygen content is high. Hope this makes sense...just my 2 cents worth