how much live rock should be put in a 58 gallon tank with a bimac and how deep should the sand be?

Sand should be about 1 inch deep.

I have about 100 lbs. of live rock. Most is base rock though.

Just make enough caves, and about 1".

I like a bit more live rock...100-150 lbs would be ok...makes the tank much more interesting for you and the octo!
Agree with the sand bit...not more than an inch or so!
Greg

I have to add my experience here. I have a 47 gallon tank with sump and bought more than 50 lbs of live rock to start with. Later on, I added 7 or 8 more pounds to fill a corner.

I ended up having too much live rock. My bimac Ollie grew bigger and started pushing out rocks into her sandy beach area, which she did use. She just shoved rocks out to make her den bigger and bigger. She also toppled the taller rocks. As I redo my tank now, I am removing rock because I realize I had too much. It was fine for a young octo but not for an adult. The sandy beach area is important for a bimc - it's an area for play and for catching crabs. I don't want to lose it the next time.

Nancy

Good point Nancy. I have a sandy area with no rock in my tank.

i plan on using 75 pounds 1/2 in the tank and half in the sump.will this be ok??

That depends, what's gonna be in the sump?

protien skimmer and mechanical filter

Well then I would put in your tank and not the sump. Unless you just dont have room. :)

if i put it all the rock in the tank will i ever see him?

Theyre more likely to be seen with rock because they feel more comfortable, they don't like coming out without a rock to hide in every few inches.

The caves of the octo make it feel more secure, it acts as a spot for the octo to dash into in the face of danger. Nick is right, the octo is more likely to come out with live rock.

thanks