[Chad Vossen]

Very likely the cause of the fish dying is ammonia. Your larval tank water may have had ammonia and low ph. At a low ph, ammonia is in a non-toxic form. Phyto has a very high ph because there's almost no Co2 in the water. When you add phyto quickly to a larval tank with low ph, you can cause all the ammonia to turn toxic, killing the fish quickly.
For this reason, I always drip phyto into the larval tanks.