Also is ther any way to take crushed coral out of a tank and put sand in if there is already fish and live rock in the tank?
Yes, I did it and so did a few others. A pain, but I think it paid off in the long run. I would suggest doing 33-50% of it at a time. This will allow the beneficial bacteria and other substrate critters time to move to the new sand. I did mine 1/3 at a time, just to be safe. I moved my LR
to one side and used a plastic strainer. I also did a 30% PWC
You don't have to have any kind of substrate. This will depend on what you want in your tank. If you want a critter that lives in the sand, then you will need a sand substrate and so on......