Don't wait to add fish if you use Bio-Spira. Not 100% sure, but it's basically just the bacteria in a pouch that are dormant because they're refigerated. When you put them in a warm tank, they need something to eat to blossom. If you don't provide them with food (waste), then they will mostly die off and when you add fish you'll basically be doing a cycle with the fish. The best thing to do is set up your tank they way you want it with conditioned water and let it run. Decide what fish you want to stock it with and get them and the biospira and add them the same day. You don't need to work up to adding fish with biospira, you can stock everything you want at once. The biggest drawback to this is that it will cost you from 10-15 dollars for enough for a 20 gallon tank. If you don't want to spend that, I've also used the method of putting an established filter on a new tank. With this method, you should also add fish soon after putting it on because you'll lose bacteria if you don't.