aaronhill_cincinnati

How many pounds of rocks can I safely put in a 75 gallon tank?

cerianthus

If tank can hold 75 G of water (approx 640lb) I guess can hold same Only if weight of the rock can be distributed equally on the bottom as water would. Impossible to do so with the rock though.
As long as rocks being added to water is not piled up on one side of the tank, I believe the weight of the rocks will just displace the weight of the water lost (volume displacement)as long as you are not adding boulders (denser than water/volume).

On Reef tank, we usually add anywhere from 1 - 2 lbs of Live Rocks for gallon of water.
For 75, this would be anywhere from 75 lbs to 150 lbs in the tank w/o mishaps.

Try NOT to exert just on one side when adding the rocks. I personally have added lot more rocks for African Cichlid (180 G tank w/500-600lbs of Coral Rocks; very dense rocks) without mishap. Not saying you should add that much but 1-2 lbs/gal, evenly distributed should be ok.
Hope this helped bit.

jdhef

I think that rocks are heavier/denser than water for a given volume (hence that would explain why they sink), so don't think that you can depend on the displacement idea as being valid.

That said, I really have no idea what your tank can hold weight wise. Maybe you could contact the manufacturer.

cerianthus

Not all the rocks may be denser than water. Porous rocks such as Lava rock, Premium LR for salt, for instance may not be as dense as water. That is what I have said I thought..
Thus it all depends on how the weight is distributed. I dont think anyone will try to fill the tank with rocks leaving no space for fish to roam about. Other than boulders (used to have imported Lake Malawi Boulder, Malawi live rock, which was very dense), should be ok provided weight is well distributed and as long as not more than total weight of water.