I have no idea what Mega Shock is or what's in it, but I do know that if you got it from the pool store, you paid way too much. Adding algaecide isn't part of the process so I don't know why they had you do that.

I think you have to go cold turkey. Cut the strings from the pool store and follow the directions of the experts here. 1) They know what they're talking about. 2) They're not trying to sell you a bunch of unnecessary and expensive products.

Read this entire thread and follow their advice. Use plain old Chlorox bleach. Keep your water at shock level; test the water numerous times through the day; keep the filter running non-stop; backwash often.