Why do I have algae, even though my chlorine levels are correct?

[Cattate21]

Why is there alge growing in the pool when the chlorine levels are in the normal range?

[PoolDoc]

There are two common reasons. The most likely, is that you have the wrong idea about what the "normal" range is.

One of the most important concepts I began teaching here -- almost 20 years ago now -- is that the correct chlorine range is determined by your stabilizer level. If you don't know your stabilizer level -- and test strips are PARTICULARLY inaccurate in measuring CYA -- you CANNOT know your correct chlorine range!

Monsanto Chemical, who developed dichlor and trichlor and promoted the idea that stabilizer can be used to reduce chlorine loss in pools -- all good things -- also promoted a very deceptive study, done in the 60's I believe, that was used to 'prove' that chlorine levels in pools with stabilizer should be the same as chlorine levels in pools without stabilizer. This was simply not true.

By the late 90's, I'd worked out that it was not true, and began explaining that on PoolSolutions.com and later, here. Beginning around 2005, Richard Falk (posting here and elsewhere as "Chem Geek" worked out the analytical basis for my observations, and developed detail calculations to determine exactly what the relationship was between CYA levels and chlorine levels. It turned out that my empirical "Best Guess Chart" was very close to the analytical values.

So, with all this proof, and with the field testing and validation of these ideas by 100,000's of users here and on TroubleFreePool, the pool industry did an about face, and acknowledged the errors they'd published in their label data, in their reports to the EPA pesticide regulation unit, and in the study programs, like the Certified Pool Operator program.

NOT!

What you have to understand is, the more screwed up your pool, the more profitable chemicals pool stores can sell, and pool companies can manufacture!

Your fastest option is to go to Walmart and get a HTH 6-Way Test Kit. The Walmart near you shows them in stock. Or you can order from Amazon or Walmart.

Once you use the kit to test your CYA level, your NORMAL chlorine level should be 5% of your CYA level, but once you have algae, your level should be 15 - 30% of your CYA level. Read the instructions CAREFULLY. That kit only contains enough reagent to test CYA three times.

You'll also need to order a K2006 kit from Amazon. (Remember, pool store testing is designed to SELL chemicals, not SOLVE problems!)

Anyhow, that's the most likely reason.

But there is another possibility.

Algae is a plant. Fertilizer makes plants, like grass OR algae, grow. Lack of fertilizer doesn't kill them, but they grow poorly. For some complicated reasons, the fertilizer that has the most 'control' in pools is phosphates.

Phosphates get into pools in at least three ways:

1. Almost all pool store 'stain control' agents either ARE phosphates, or TURN INTO phosphates over time (they start as phosphonates) Many times, pool owners will walk out of pool stores with BOTH a phosphate remover AND a stain-control product that ADDS phosphates!

2. Water companies add phosphates to drinking water (at harmless-to-people levels) to help protect old iron or steel pipes. The amount added varies from time to time, not following any pattern I can recognize.

3. Most body fluids, but particularly pee and sweat, contain enough phosphates to have an effect. And ALL year-round competitive swimmers habitually pee in pools -- it's not just Ryan Locte.

This is a solvable problem, but not one pool stores ever seem to solve. Again, the reasons are complex, but boil down to the fact that poor testing and poor phosphate removal products INCREASE their profits. Also, while lowering phosphates is easy enough, it requires paying regular attention to your pool's chemistry, something many pool owners struggle with.

In your case, start with the chlorine levels. Get them right, and that may be all that's needed. If your stabilizer is too high, Walmart has begun selling 10% pool bleach as Pool-Essentials-Chlorinating-Liquid. Use that, instead of dichlor or trichlor, since it will not add more stabilizer.

However, if you'd prefer to pursue both possibilities at once, you'll need to ALSO order the Taylor K-1106 phosphate test kit.

Good luck!