I was asked why my breeder tanks do not contain gravel.
The answer is simple.
All that brine shrimp pollutes the water very quickly. When there is a large group - angelfish can lay around 300 eggs, though all of them won't hatch - the tank can become very dirty very quickly. One of the keys to an angelfish's health and chance of survival is clean water.
By keeping the tank free of gravel, it makes it easy for me to clean it thoroughly without running the risk of sucking up any of the baby fish.
I learned this after setting up an original breeder tank with gravel (shown in this picture.) Each of these baby angels were about three weeks old when this picture was taken, and they are only the size of a piece of gravel. I was very careful with cleaning but it took much longer to clean than a regular tank because I had to go very slowly to make sure I wasn't harming any of the babies.