Q: Hi Rob. I have been reading your articles for a few weeks, and find them pretty interesting. We are long time pool owners. I thought I was pretty in tune with my pool, but now something weird is happening. We are starting to get a strange purple stain all over the walls and on our pool cleaner. It feels a little like a calcium deposit, but its purple. Have you heard of anything like this before? I tried to Google some information it, but I’m not having any luck. I’ll keep my eye out for a response. Sure hope you have an answer. Thanks! Robert G.

A: Robert I saw your email and responded right away. Yes, purple staining is definitely unsightly! I am very familiar with purple staining, but it’s not a common issue most pool/spa guys come across. This is the reason you had a difficult time researching the topic.

The actual issue is called copper cyanurate. Forgive me for the fancy word as I wear my lab coat. An explanation will work better. 3” chlorine tablets have an ingredient added to protect the chlorine from loss due to the suns UV rays. Overtime the level of this ingredient builds in the pool.

At some point everyone gets algae in their pool. The active ingredient in pool algaecide is copper. In most algaecides, the copper is not treated to keep it from binding to the ingredient in the chlorine tablet. Copper in the algaecide only binds once the level of the chlorine additive reaches a certain point.

The reason why you’ve never encountered this issue before, is because the additive level was never high enough to cause a reaction. A citric acid additive will reverse the staining.

Rob Robinson