You can either buy the special removal agent from a store or make your own removal agent which is easy (the chemical companies don't want you to know this). All you have to do is 1.take some water and add muriatic acid, 1/2 the amount of water, (should be abundant in whatever pool storage room) (and always remember to add chemicals to water, never the other way around). 2. Add chlorinating granules or liquid chlorine (either is fine, depending on what your pools uses) until you begin to smell a strong odour. 3. Mix thoroughly and then scrub the pool tiles with it (you will have to drain the pool though).