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The 5L of Ultima-Plus XP Moss, Mould & Algae Killer is cheaper than most products featured in this review. Another one marked for outdoor use, it is biodegradable, non-caustic and therefore is safe to use all around. Like most algae killers in this category, you have to wait till it is completely dry for it to be safe for skin contact. You can dilute it as you please to achieve the best results but work within safe limits. It offers you value for your money and you get to enjoy good results so why not?