Technically, BB live off ammonia, not fish, so you don't need fish, but I know what you mean. You do need a source of ammonia, unless your tank already has ammonia.
It doesn't matter how old the tank is....if the BB ran out of ammonia, they all died not too long after, unless you kept the tank temp down around 40 F. If you were feeding fish there within the last week and want to keep the BB alive, just add the appropriate amount of ammonia every day to feed them.
You can keep BB alive that way indefinitely.