I put rock rubble in the filter of my 40 gallon tank and used it to seed my 20 gallon. I never had any ammonia reading, the uglies started 1 day after setup, first fish added 2 days later and I changed the water twice a week for a month. I added 2 more small fish once a month. Just make sure you understand that the bacteria will match the food source. If you add too much at once, even if you have bacteria, it won't be enough to keep up. This is why I added a fish so quickly to my 20, otherwise most of the nitrifying bacteria would have died off due to lack of food.