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[i have crabs]

enough rock for what?
if your using it for filtering that depends on the bioload you plan on keeping
if it mostly for looks then i say add more but thats personal preference
its hard to say but i would guess you have way less than 30lbs i would say closer to 15-20, i have about 80lbs in my 60g and probably 500+lbs split between 2 other tanks but i like the look of a tank full of rocks