I have algae...

[Kabong]

You can use a argonite based sand which will help buffer your tank, This is your most often used sand. Usually bought under the Carib-Sea brand name at your LFS
Or you can use a silica based sand which wont buffer but is generally cheaper. It's not used as often and you must make sure that it's a pure silica based sand. You can get it at garden shops, pool shops, or Home depot.

All my tanks have silica based sand in them so i cant really comment on the "buffering" effect of argonite.

Yes changing out the crushed coral will probably start a small cycle again, which is why its best to do in now.