Hi Roger, your fine i wont take it the wrong way, but Colin is a fellow CIU member and if he wants to maintain his indian sandstone there are other solutions than having to use Hypo based products which i am sure you will agree are not the safest products to be using, the product i was referring to is a maintenance product and once sprayed will keep algae at bay for six months, when it can be sprayed again to keep it at bay for another six months and so on. this is only a suggestion under no obligation and Roger i understand why you would not want your clients to know about these types of products.