Pool chemistry basics for new owners

A new owner can get lost in test kit colors. Start with two numbers and learn the rest as you go.

Free chlorine is the chlorine available to kill germs and algae. CDC recommends at least 1 part per million in a pool, and at least 2 ppm if you use cyanuric acid or stabilized chlorine products like trichlor tablets or dichlor. Most owners aim a bit higher than the minimum so there is a cushion when the sun or a pool party burns it off.

pH measures how acidic or basic the water is. CDC recommends 7.0 to 7.8. Its reasoning is worth remembering: as pH rises, especially above 8.0, chlorine loses much of its ability to kill germs. As pH falls, especially below 7.0, chlorine works faster but the water starts to corrode equipment and irritate eyes and skin. Most pool pros aim for the middle of that range.

The supporting numbers change slowly and are tested less often.

• Total alkalinity is a buffer that keeps pH from swinging. The Pool and Hot Tub Alliance points to the industry standard range of 60 to 180 ppm, and says 80 to 100 ppm is ideal when you use liquid chlorine, cal-hypo or a salt chlorine generator.
• Calcium hardness is how much dissolved calcium is in the water. The same industry standard treats 150 to 1,000 ppm as acceptable. Too low and water can be aggressive to some surfaces. Too high and it scales.
• Cyanuric acid, or stabilizer, protects chlorine from sunlight. PHTA gives an ideal of 30 to 50 ppm and a maximum of 100 ppm. It builds up every time you use trichlor or dichlor, and the only practical way to lower it is to replace water.

More often than most people do. CDC's guidance for home pools is to test chlorine and pH at least twice a day, and more often when a lot of people are swimming. That sounds like a lot until you realize a strip takes thirty seconds. At a minimum, test before anyone swims on a hot day and after heavy use or a storm.

Test alkalinity and calcium hardness every week or two, and cyanuric acid every few weeks if you use stabilized chlorine. Take a sample to a pool store or have your service tech check the full panel at opening and a few times through the season.

CDC calls a DPD test kit the most accurate option for chlorine and pH, with test strips handy but less accurate. One quirk: very high chlorine, above about 10 ppm, can bleach the test and read falsely low or zero. If you just shocked the pool and the test says zero, wait and test again rather than adding more.

There are a few common ways, and they affect the other numbers differently.

Stabilized tablets, usually trichlor, go in a floater or an inline feeder. They are convenient and add cyanuric acid every time, so stabilizer climbs through the season. They are also acidic and pull pH and alkalinity down. Liquid chlorine and cal-hypo shock add no stabilizer. Liquid chlorine raises pH a little, and cal-hypo adds calcium. A salt chlorine generator makes chlorine from salt dissolved in the water, which many owners find gentler on skin and easier day to day. Salt systems still need pH attention, because they tend to push pH up.

Whichever you use, shock or superchlorinate after heavy use, after a storm that dumps debris in the pool and at the first hint of green. Our starter chemicals get you through startup, and we walk you through your sanitizer choice then. If you want a salt system, it is one of the upgrades we install.

Usually the opposite of what people assume. Clean, well-chlorinated water has very little smell. The sharp "pool smell" mostly comes from chloramines, the compounds that form when chlorine reacts with sweat, sunscreen, body oils and other things swimmers bring in. That used-up chlorine is called combined chlorine, and it is poor at killing germs.

A DPD kit can show it. Test free chlorine and total chlorine, and the gap between them is your combined chlorine. When that gap grows, or the pool smells strongly and eyes sting, the fix is a proper shock dose that burns the chloramines off, plus good circulation. Showering before swimming cuts down on what forms them in the first place.

Northeast Ohio pools deal with a few regional swings.

Rain. Summer thunderstorms can put a lot of rainwater into a pool in an hour. Rain is slightly acidic and has no chlorine, alkalinity or calcium, so a big storm dilutes the pool and can nudge pH. It also washes pollen, dust and leaf litter in from the deck and trees. Test after any heavy rain and expect to add chlorine.

Pollen and leaves. Spring brings tree pollen and seeds, often right around opening, and fall brings leaves. Organic debris uses up chlorine as it breaks down. Keep baskets empty, skim often and keep the pool covered when it is not in use. Our guide on trees and pollen goes into more detail.

Temperature. In May the water is cool and chlorine lasts. By late July, with warm water, long sunny days and more swimmers, chlorine demand climbs. The amount of chlorine that was plenty in June can fall short in August.

Opening and closing. Opening day usually needs a big correction after a winter under the cover. Closing needs balanced water and a good chlorine level so the pool does not open green. Both are part of our pool service if you would rather not do them.

Make one change at a time, in the order that matters. Alkalinity first, because it steadies pH. Then pH. Then chlorine. Calcium and stabilizer only when they are out of range.

Add chemicals in small doses with the pump running, wait for them to circulate, usually several hours, and retest before adding more. Most bad pool days start with someone doubling a dose because the first one did not show up on a test five minutes later. Know your pool's volume in gallons, because every dosing chart on the bag is written per gallons of water.

Keep a log. A notebook or an app with the date, readings and what you added turns chemistry into a pattern you can see. After a season you will know how much chlorine your pool uses in July and how the pH drifts after a storm.

Pool chemicals are strong oxidizers and acids. They are safe when handled right and dangerous when mixed. The New York State Department of Health's pool chemical guidance is a good, plain list, and its case examples, including a homeowner who mixed shock and acid in a kitchen pan and caused an explosion, make the point.

• Never mix chemicals with each other, even different types of chlorine.
• Add chemicals to water, never water to chemicals. Add acid to water, not the other way around.
• Use a separate, clean scoop for each product.
• Store chlorine and acid apart, dry, cool, off the floor and locked away from children.
• Keep lids tight so moisture cannot get in.
• Wear eye protection and gloves when handling them.
• Do not add chemicals while people are in the water.

If a container is leaking, smoking or giving off strong fumes, leave the area and call for help rather than trying to clean it up.

Not much, which is one of the things owners like about it. A fiberglass pool has no plaster to etch and no liner to fade, so it tolerates the ordinary ups and downs of a home pool well. Keeping pH in range and calcium from running high is still what prevents scale lines at the waterline and deposits on equipment, and balanced water is kinder to heaters, pumps and salt cells whatever the pool is made of. Follow the manufacturer's water care guidance for your shell, and ask us if you are unsure.

You do not have to become a chemist to own a pool. Many owners learn the routine in a few weeks with a good test kit and a log. Others would rather hand it off. Our pool service includes openings, weekly cleaning, water testing and chemical balancing, equipment checks, repairs and closings, so you can take on as much or as little as you like.