When starting up a new saltwater tank how long do you wait…?

You can add both as soon as the salt is well mixed (a few hours at best) and the water is up to the correct temperature.

Since you added the sand first, you’ll need to move it around when you add the rock – the rock should sit directly on the bottom of the tank and be placed and stacked so it doesn’t wobble. There are fish and invertebrates which are natural burrowers, and this keeps your rock from toppling over if anything would get under it.

Since the point of having live rock and live sand is biological filtration, you don’t want to have it in your tank for too long without an ammonia source – this is what keeps the bacteria alive and reproducing. Until you know that the bacterial population is sufficient to convert any ammonia and nitrite to nitrate, you’ll need to add something to be an ammonia source – a piece of shrimp, or a few shrimp pellets – these will also serve as food for the microorganisms on the rock. Once you see your water tests reading 0 ammonia and nitrites, you’ll have some variable nitrate reading, do a water change if necessary to bring the nitrate level below 20ppm, then you’re safe to start adding a few fish and "cleaner" inverts like snails and hermit crabs if you want these. Wait until the tank’s been running for about 6 months before adding starfish, corals, or anemones.

Wait 2-3 days add your live sand your water will most likely turn cloudy wait for it to clear up and add your live rock.