The 1 - 1.5 lb / gallon is to have effective biofiltration. You could have less, but have to rely more heavily on other filtraion methods. In a fish only tank, you could get away with no LR at all, but would have to rely heavily on a wet/dry, canister, or other power filter for filtration. In a reef tank, it would be realy difficult to maintain a successful reef without an adequate amount of LR (and much more expensive to maintain vs. having the LR).