It's all about porosity.
If you have base rock with lots of pores in it, it will still be a decent biofilter...
What makes actual live rock (or dead coral fossilized) so good for a biofilter is that it is exceptionally porous, with the more expensive rock usually being congruent with how light it is (light being an indicator of how porous or empty it is on the inside).
For a good biofilter, you need not only surface area, but also area where water can move but no oxygen can get to inside the rock.
If you don't have enough of both of the oxygen rich and oxygen poor surface area interacting with your water, you won't be able to get the proper bacteria forming to filter it.
Base rock in general is not very porous, especially inside where the oxygen is poor...
so you can wind up with a pretty half ass biofilter if you skimp on the LR...
I've had LR for sale at 3$/lb for some time, the average used price seems to be around 3.50$/lb, how much cheaper are you hoping to find it for?
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