Need a good patio cleaner

[WendyS]

Our yellow paved patio slabs are black and grimy and covered with algae. Can anybody recommend a really effective liuqid patio cleaner? I don't want to power wash them again as previously using this method has removed all the protective smooth coating on the slabs and left a gritty surface which only attracts grim and lichen more quickly, even though we put a sealant on afterwards. Thanks.