Could have been number of things...
1. cleaning up to the edge of the sand bed didnt hurt anything
2, whenever you chage salts....try to "ween" your tank from one to another...Its alway a good practice to slowly phase out one salt if your going to change. "daveinz" was right.....salts have different chemical properties.....some things can be sensitive. UNderstandable that you just got caught short...errrg
As far as water changes....is sounds like things are coming back around. I would suggest doing the same WC on the sam schedule....more drastic changes can be harmful
Hope this helps
GT