I worked in the swimming pool industry for 16 years here. I was driving around with quite a few of those banned substances in the back of my van. Hydrochloric acid, oxalic acid, hydrogen peroxide. They are all available off the shelf in pool shops in as large a quantity as you like. Although, in a country which doesn't really have rusty cars I managed to rot the floor out of two vans. Its just as well the UK is to cold for swimming pools cos you cant buy pool products.