There is also a non chlorine oxidizer (it is NOT MPS) that has been suggestged for use by the company that makes it as a non chlorine shock. Don't know too much about it , but it is sodium percarbonate. Really don't know what effect it has on the pool but it is supposed to complete the conversion in abut 48 hours. I do know that there are problems with other non chlorine, non MPS oxidizers in pools such as sodium perborates. Can't find much info about it and the maunufacturer's website is offline! I beleive Ben might know the manufacturer of this product and have more to say on the issue. In the meantime I would stick with Bleach!