Pool and Spa
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Hi if your confident you have chlorine in the pool, there could be two reasons why the DPD isnt showing.
The first could be due to the tablets, they may have expired through damp or age.
Secondly it could be due to what is known as Chlorine bleaching, where you have so much chlorine in the pool its bleaching the colour, to check this take a 10% dilute of pool water with distilled water (tap water will suffice) then test it, if the colour returns this proves its due to chlorine bleaching, and you just need to reduce the chlorine level.
The green stains on the liner are probably caused by algae, which is often due to low chlorine or high PH, please check out some of the hints and tips here.With regard to removing the stains I would look at an Algae cleaner such as the one here, or try and use some dilute acid, the challenge you will find is if the Algae has built up over time it has probably taken the protective layer of the liner away and bedded in so your not really only bleaching the green colour out of the liner.Please let me know if you need anything clarified.