argonite is the way to go if you can find it cheap. I couldn't so I had to buy it at my LFS
. Silca Sand will not hurt your tank, But it won't help either. Some tiny critters may have a problem with it.
You will need more sand than what you have in there currently. I have a 37 gallon tank with 50 lbs of sand, and I dont think thats quite as much as I should have. Use this calculator: http://www.aquariumadvice.com/calcs2.php?type=sanddepth
To determine How deep you need it. Then I would say go spend the 25 bucks at your LFS
and get Argonite Sand to pile on top or your existing sand. I say If your gonna do something, you might as well do it right! (but thats just me)
(plus, my argonite sand from the LFS
came with tiny little shells that look just awsome poking out of the sand in my tank.)
and Yes, I would remove the Crushed Coral.