Live phytoplankton not only keeps uneaten rotifers alive in the fry tank to make them constantly available to fry but also keeps the fry tank water clean by taking up ammonia, nitrate, and phosphate. For the first few days when your larvae are so small you cannot use a sponge filter and the waste accumulates fast. The ammonia level in the tank goes up really fast within four days. It rises even with live phytoplankton but at much slower rate. You can do daily water changes with the water from the parent's tank to keep ammonia down. Amazingly large water changes do not bother clownfish fry. When I was breeding FW angelfish fry, I used to do a near 100% water change daily for my fry just like many other breeders do. Clownfish fry are just as tough.
HTH,
Tomoko