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Test your pool's fill water. Customers depend on it.

March 22, 2017


Wistia video thumbnail - All pool chemistry starts at the tap

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Pool professionals need to test every customer's fill water

Let's start by defining the term 'fill water', because some people may call it 'tap water' or 'hose water'.  "Fill water" simply refers to the source water that fills a pool. It is wise to assume everyone's fill water is different, even if they are in the same neighborhood. Without understanding its water chemistry, we pool professionals have no baseline of understanding.  This is especially important for new plaster startups and re-openings for the summer season.

Our opinion at Orenda is simple: customers depend on pool professionals to do the right things and treat their pool properly. Testing fill water is a good start to delivering consistent results to that customer. Think about it...water evaporates out and constantly needs to be refilled; if we don't know what the chemistry is, how do we know what to put in the pool?

We don't.

Things to test for, and why

  • pH. Whether we like it or not, pH likes to fluctuate in a pool. Normally, water out of the tap is slightly alkaline, which has a direct impact on chlorine efficiency. If our fill water is acidic for some reason, that will have an effect on the pool's pH too. pH also affects chlorine efficiency and the LSI. Pool additives later can correct this, but only if we know what to correct.
  • Alkalinity. Read more about the relationship of pH and total alkalinity here. Measuring alkalinity is very important for pool operators.


    test fill water to prevent etching If the fill water has low calcium, the pool water will draw calcium from the plaster surface,
  • causing permanent damage (etching).


  • Calcium hardness. This one is major. We were at a pool recently, and we tested the pool chemistry. It had a calcium hardness reading of about 250 ppm. Yet the pool surface was clearly etched and pitted. It looked green and awful. So we tested the fill water...and just as we suspected, the calcium levels were far below 250. The tap water was 140 ppm.  Is it any wonder how the hardness level in the pool climbed up from 140 to 250 ppm?  Nope. The plaster surface donated the calcium to the water, because water seeks equilibrium. Measuring for calcium hardness is critical for new startups to protect plaster, but also for ongoing pH buffering and surface protection.
  • Metals. Most pools get their water from municipal treatment plants, which send water through a series of pipes. It is not uncommon for some of this infrastructure to be metal, which can be dissolved into the water stream. In an extreme case, think of the water crisis in Flint, Michigan. Water is the universal solvent, which means it can pull metals into solution. For swimming pools, the metals most commonly found are copper, iron, manganese. If and when they over-saturate, these metals can fall out of solution and stain your pool. We recommend using a metal trap on the end of the hose to capture those metals before they get in your pool. Even pools that are filled with water from a delivery truck can have issues.
  • Phosphates. Testing for phosphates is directly related to the previous two items: calcium and metals. Why? Because in the past decade or so, municipalities around the country have been putting phosphonic acid into drinking water as a sequestering agent. They do this for many reasons, which you can research on your own...but the point is, phosphates are often introduced to pools via the fill water. Depending on where you live, well-water may also be high in phosphates.
  • Free chlorine. Would you be surprised if we told you that tap water can have 1.5 ppm free chlorine in it? Well, it's true. We test water all over the place and have seen it. 1.5 ppm is a great number for commercial pools to operate on! Yet we drink it. Bottom line, test chlorine levels.
  • Combined chlorine. While many places add free chlorine for disinfection, others add chloramines to their water supply. Why? Because chloramines react slower in the disinfection process than free chlorine, and can carry through the pipes longer to still be present in the water at the tap. We encourage you to research this on your own, but it is not uncommon for pools to be pouring in fill water that has combine chlorine levels over 0.2 ppm. Can you say "air quality problems"?
  • Total dissolved solids (TDS). Especially in areas like Southern California that often restrict drainage, knowing the TDS makes a big difference for old, rarely-drained pools.

Baseline testing can protect you and your customer

chloramine pool test kitAnyone working with pools should test fill water before doing anything else. This goes for construction companies, pool designers and especially operators. Why designers? Because knowing the base chemistry of the source water can help determine what type of equipment and chemicals to use. For example, if the source water has very low calcium hardness, cal-hypo tabs may be a good choice for that pool. If not cal-hypo, another pool additive like calcium chloride.

For construction companies and plaster finishers, we need to know what's in the water because it immediately affects the plaster in its most vulnerable time: the curing process. We recommend using the National Plasterer's Council (NPC) startup guideline (available here). The NPC indicates the need for testing the fill water, and adjusting alkalinity, calcium and using a startup sequestering agent to protect the surface while it cures. If we do nothing else during startup, let's at least test calcium first, and add it ASAP before the plaster donates it, at the cost of permanent etching.

Customers deserve proper treatment of their pool

The only way to deliver the care and treatment our customer is paying us for is to understand the water we're working with. No exceptions. It's easy to do, and we industry professionals owe it to our customers and their pools. Protect yourself and your customer by testing water, and making you adjust the chemistry to be within a safe range on the LSI. Need help with the LSI? Download our mobile app with LSI calculator.


Wistia video thumbnail - New pool startup with LSI Calculator and SC-1000

Thanks for reporting a problem. We'll attach technical data about this session to help us figure out the issue. Which of these best describes the problem?

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