The Ultimate Guide to Water Chemistry

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Have you ever struggled with chemical levels in your swimming pool?

Have you gotten an algae bloom despite constantly adding chlorine? Or let your CYA get so high you needed a partial water replacement?

You are not alone! The good news is once you understand how all the chemicals in your pool work together, you can avoid these problems altogether.

We’ve put together The Ultimate Guide to Water Chemistry to help you get control of your pool. We’ve gathered information from our experts here at PoolSupplyWorld and TFP (our favorite online pool forum). Enjoy!

First things first, these are the five chemical levels that are most important to understand and keep track of.

  • FC (Free Chlorine) A sanitizer that keeps your pool water safe and germ-free. Even in salt water pools, Chlorine is the sanitizer that prevents algae blooms. Chlorine must be constantly replenished. The ideal level of FC depends on your CYA. The higher your CYA is, the more FC you need.
  • CYA (Cyanuric Acid) Acts like sunscreen for your FC (prevents burn off from the sun), but also lowers the effectiveness of FC. The higher your CYA, the more FC you need to prevent algae blooms. Ideal levels are 30-50 for outdoor pools (no SWG), 70-80 for SWG pools, and 0-20 for indoor pools.
  • PH (Acidity/Alkalinity) Needs to be balanced to avoid skin irritation and protect pool surface & equipment. The ideal range is 7.5 to 7.8
  • TA (Total Alkalinity) Helps keep PH in balance. High levels can cause PH to rise. The ideal range is 60 to 120
  • CH (Calcium Hardness) Help prevent plaster damage when balanced correctly. High CH can cause calcium scaling. Ideally 220 to 350 for Gunite. Lower for pools with a vinyl liner.

Want to get your levels checked and balanced?

We recommend the TF-100 Test Kit!

tf100 pool water test kit
There are four other chemical levels that you should be familiar with:

  • CC (Combined Chlorine) This is chlorine that is actively fighting algae or bacteria. When a pool smells like chlorine (public pools, for example), you are actually smelling CC, not FC. Combined Chlorine over 0.5 indicates a problem.
  • Borate is an optional enhancement that can help maintain pH levels. If you are using Borates, 30-50 ppm is ideal.
  • Phosphates are like food for algae. You can remove phosphates from your pool to help keep algae in check. However, this is optional because chlorine (alone) is enough to prevent algae outbreaks. It’s simply a matter or preference.

Below is an in-depth description for each of the 9 chemical levels.

If you want to adjust your levels, use the PoolMath Calculator!

FC  (Free Chlorine)

Free Chlorine is what prevents algae from blooming in your pool. If you do get an algae bloom, FC attacks it and kills the algae. Maintaining an appropriate FC level is critical for keeping your water balanced and clear. If you allow FC to get too low, you run the risk of an algae infestation. It is also unsafe to swim in a pool that has no FC. Ideally FC should be tested daily to keep your pool sanitary and clear. If you have a SWCG (Salt Water Chlorine Generator), you can test for FC less often (every few days). FC is depleted by sunlight, and used to break down organic material. The perfect level of Free Chlorine depends on your pool’s exposure to sunlight and CYA level. The higher your CYA, the more FC you need in your pool. If you want to raise the FC of your pool, we recommend you use liquid chlorine, tabs*, household bleach, or a SWCG.

*Tabs contain small amounts of CYA. Although the CYA is a very very small dose, be sure to test your CYA regularly if you are using tabs as your chlorine source.

CC (Combined Chlorine)

Combined chlorine is the chlorine that is actively working to breakdown organic matter in your swimming pool. CC is what causes the smell associated with chlorine. So when you can smell chlorine in a pool, that means FC is bonding to organic matter or contamination. If CC levels get above 0.5 ppm (parts per million), it is recommended that your SLAM** your pool. is an intermediate breakdown product created in the process of sanitizing the pool. In an outdoor pool, CC will normally be zero, as long as you maintain correct FC level. Potassium monopersulfate (a common non-chlorine shock) will show up on FAS-DPD chlorine tests as CC. There is a special reagent you can get to neutralize the potassium monopersulfate so you can get a true CC reading.

**SLAM stands for Shock Level And Maintain. The term was created by the TroubleFreePool community to identify the process it takes to clean a pool with an algae bloom. In the past, the process was called “shocking” your pool, but that was confusing because “shock” is also a product. In short, SLAMing your pool involves hyper-chlorinating it to kill any organic matter that is in your pool. This process normally takes several days, but is a necessary measure to take when you get an algae bloom.

CYA (Cyanuric Acid)

Cyanuric acid is commonly called stabilizer or conditioner.  CYA acts like sunscreen for your Chlorine. It protects your FC from burning off (from the sun), but also lowers the effectiveness of FC to fight algae. The higher your CYA is, the more FC you need to keep algae at bay. For this reason, it is critical that you test CYA often to avoid an algae bloom. Ideally you should keep CYA between 30-50, assuming you do not have a SWCG. If you do have a Salt Water Chlorine Generator, or your pool is exposed to lots of direct sunlight, you want to maintain a higher level. The recommended CYA level for SWCG pools is 70-80.

You raise CYA by adding cyanuric acid (also called stabilizer or conditioner). The liquid CYA costs more, but will register on your water test shortly after adding it. Solid stabilizer can take up to a week to fully register on the test, so don’t retest your CYA level for a week after adding some. Solid stabilizer is best added by placing it in a sock in the skimmer basket. Run your pump until all of the powder CYA is dissolved, and continue to run the pump for an additional 24 hours. Avoid backwashing and cleaning your filter for at least a week after adding solid CYA.

Lowering your CYA is tricky, so it’s important that you do not let it get too high. To lower CYA, you will need to do a partial water replacement or reverse osmosis.

pH (Acidity/Alkalinity)

pH indicates how acidic or basic your pool water is. When you first get a pool built, it is important to test pH daily to protect the pool surface. If you have fresh plaster, it is common for pH to drift up over time. pH will also rise when your pool is aerated (from rainfall, spa, waterfall, etc). Once you get a good sense of your pool levels, and how fast your pH drifts, you can monitor the pH less frequently.

Ideally you want pH between 7.7 and 7.8, but 7.4-7.8 is fine. pH below 7.2 will cause your eyes to sting, and pH below 6.8 can damage metal parts of your pool equipment. High pH can cause calcium scaling.

To lower your pH, use muriatic acid (pH Reducer). To raise pH, use borax or soda ash (pH Increaser).

TA (Total Alkalinity)

Total alkalinity provides a buffer for pH changes. At low TA levels, the pH tends to fluctuate often and easily. At high TA levels, the PH will normally drift up.

To raise TA, you can use baking soda or Alkalinity Increaser

Note: If you want to make major adjustments to your TA, it is best to do it in phases, testing after each one.

To lower your TA, add acid to lower your pH to 7.0 to 7.2, then aerate your water. You can aerate your pool using a water feature. If you don’t have a water feature, you can aerate using a sprinkler attached to your hose (splashing water into pool). Repeat those steps until you reach desired TA.

Salt

Salt is required with a SWCG. The SWCG converts the salt into chlorine to sanitize your pool. Check your SWCG manual for the correct salt level for your unit. The level will likely be around 3,000 ppm, but different models call for different salt levels.

Even without a SWCG, salt can be added to give your water a softer feel. For improved water feel without a SWG, try levels around 2,000.

Salt can be added using solid or liquid salt.

Note: the recommended levels of salt are one tenth the level in ocean water (35,000 ppm). While some people can taste salt at 1,000 ppm, most people cannot taste salt  under 3,500 ppm.

Borates

Borates are an optional enhancement that helps control PH drift, keeps algae in check. Some people claim borates improves the water quality and feel. There is no need to measure for borates unless your are intentionally adding them. If you choose to use borates, the recommended level is between 30 and 50 ppm.

Phosphates

Phosphates are sometimes removed from the pool as a way of keeping algae in check. Phosphates act as food for algae, so removing them will hlep prevent algae blooms. Since chlorine is required anyway, and chlorine alone can keep algae in check (even at very high phosphate levels), using a Phosphate remover is totally up to personal preference.

Want to get your levels checked and balanced?

We recommend the TF-100 Test Kit and the PoolMath Calculator!

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Kevin
When Kevin’s not enjoying the water himself he’s engaging himself in the wide world of pool and spa knowledge which he passes on to our customers through producing content for our blogs and website. Outside of the pool world you can find him enjoying life with friends, family and his off the wall dog Brixton.