I also have a 20g. I put in 20 lbs of Figi rock, and it was not enough. I don't know how much you know about rock, but figi is probably the lightest rock (that is good enough to use any ways). I ended up with 25 lbs, and I'm still not happy with that much. If you are just starting the tank, I would suggest curing the rock in the water, and simply start with 25 lbs, and buy more the next day. Once you are happy with the amount, then allow the rock to cure, do a 50% water change at about the 6 week mark, and one week later, add away.