If left, fish waste will build up and become toxic. Filter Starter contains Bacillus megaterium, B. pumilis, B. licheniformis, Paenbacillius polymyxa, Nitrosomonas spp, Nitrobacter spp and Nitrospira spp. These friendly filter bacteria break down fish waste, making the water safe for fish,Add to a new aquarium to help ‘kick-start’ the filter, when adding new fish, after a filter clean/water change, or after using a disease treatment.