there are 2 different kinds of bacteria. one to convert ammonia to nitrite, and one to convert nitrite to nitrate. the bacteria that converts ammonia to nitrite will need to build up first. only then will the bacteria that converts nitrite to nitrate can start feeding on the nitrite and multiply.
nitrosomonas species of bacteria converts ammonia to nitrite, and nitrobacter species oxidises nitrite to nitrate (: if im wrong someone pls correct me haha