I was going to mention what Ryan did. I've never used it (though I did pick up a bottle the other day, just to experiment with), and you definitely need to have the fish in first, preferably for a week or so until your ammonia reaches 0.25. :)
I've used the API product with great success several times before, and for that one, once it's been opened you need to use it within 48 hours, otherwise it's no longer effective.
I figure with all these things, it can't hurt, so if they're not too expensive and you want to try it, go for it. :)