you gotta put something in there for the bacteria to eat or else the beneficial bacteria will die, i used guppys but you can also use pellets or any sort of food. It will help the bacteria reproduce, autotrophic bacteria (the kind you want) takes 14 days to reproduce, so i would say 2 weeks minimum. I waited a solid 3-4 when i seeded my 70 and had no problems. But since your using such an old filter and havent scrubbed it clean, you might be good to go. But if you have any really delicate fish i would wait a while with those just in case