I can see there being a relationship to it not being a fully cycled tank. While the bacteria may be sufficient in population to take on a certain amount of ammonia and nitrite they have a limit. It takes time to increase the population. Putting in multiple fish at a time can overwhelm the bacteria in there for quite some time making it necessary to do very frequent water changes until the bacteria build up to the point where they can handle all that extra ammonia that was introduced with the fish. Because of this I always advise slow stocking of tanks.
