The short version
- Free chlorine has to rise with stabilizer. At 40 ppm cyanuric acid, plan on at least 2 ppm free chlorine, and more as stabilizer climbs.
- Houston sun and weeks of 90 degree days use up chlorine fast, so summer testing should happen daily, not once a week.
- Heavy rain dilutes your chemicals and drags pH and alkalinity down. Retest after every big storm.
- Chlorine tablets add stabilizer every time they dissolve, and the reliable way to lower stabilizer is to replace some of the water.
Why summer is the hard season for Houston pools
Summer is when pool chemistry slips fastest, because everything that uses up chlorine speeds up at once. At Bush Intercontinental, the 1991 to 2020 normal high is about 94.5 degrees in July and 94.9 in August, and the area averages roughly 111 days a year at 90 degrees or hotter.
Warm water, strong sun, and high humidity give algae ideal conditions. Your pool also gets used more, which brings in sweat, sunscreen, and lotion. Add afternoon thunderstorms, hurricane season running June 1 to November 30, and June as the airport's wettest month on average, and the water is pushed off balance from several directions every week.
None of this means summer water has to be a fight. It means the routine that worked in March is not enough in July.
The numbers worth holding all summer
Test kits report a lot of values. These are the ones that matter most in hot weather, with ranges drawn from CDC guidance and equipment manufacturer specifications. Your own equipment manual may ask for slightly different targets, and it wins when it does.
- Free chlorine: the CDC minimum is 1 ppm, or 2 ppm when stabilizer is present. In practice, size it to your stabilizer level (see the next section).
- pH: 7.2 to 7.8 is the range most equipment makers specify. The CDC range is 7.0 to 7.8.
- Total alkalinity: 80 to 120 ppm keeps pH from swinging hard after rain or acid additions.
- Calcium hardness: 200 to 400 ppm. Too little lets water etch plaster; toward the top of the range, watch for scale when pH runs high.
- Cyanuric acid (stabilizer): one major salt system maker calls for 30 to 50 ppm outdoors. Texas caps public pools at 100 ppm.
- Salt, for salt pools: whatever your chlorinator's manual says, often somewhere around 3,000 to 3,500 ppm.
Sunlight, stabilizer, and how much chlorine you really have
Unstabilized chlorine breaks down quickly in direct sun. Cyanuric acid shields it, which is why outdoor pools use it, but stabilizer also slows chlorine down. The more stabilizer in the water, the more free chlorine you need for the same sanitizing power.
A useful rule from a public health code committee works out to about one part free chlorine for every 20 parts stabilizer: at least 2 ppm free chlorine at 40 ppm stabilizer, and about 4.5 ppm at 90 ppm. If your test shows 3 ppm chlorine and you feel safe, check the stabilizer number before you relax.
Watch the source of your chlorine. Trichlor tablets and dichlor granules carry stabilizer in the molecule, so every tablet raises cyanuric acid a little. Over a summer of tablets, stabilizer creeps up and chlorine works less well even though the reading looks fine. Liquid chlorine does not add stabilizer. Cal hypo does not either, but it adds calcium, which matters in hard water.
Stabilizer does not evaporate, and it does not wear off in any amount you can count on. Once it is high, the dependable fix is to drain part of the pool and refill with fresh water. On a large drain, get advice first, since lowering the water a lot can stress the shell when the ground around it is saturated.
pH drift and scale in hot water
Most pools see pH rise over a summer. Waterfalls, bubblers, spa spillovers, and swimmers splashing all let carbon dioxide escape, which pushes pH up. Salt pools tend to see pH climb steadily as well, so they usually need acid more often than tablet pools.
High pH matters more in heat. Warm water already holds calcium less readily, and when pH climbs too, calcium starts dropping out as white scale on tile, in the salt cell, and inside the heater. Evaporation makes it worse: water leaves, minerals stay, and every top off brings in more.
Houston tap water varies by address. The city's main system averaged about 135 ppm hardness in its 2025 report, with a range of roughly 44 to 229 ppm, and parts of Sugar Land's system reach about 280 ppm. Know your own starting number and test calcium every week or two during peak evaporation. If a crusty line is already forming at the waterline, pool tile cleaning removes it before it hardens further.
Rain, runoff, and topping off
Rain is not neutral water. Normal rainfall is slightly acidic, a little below pH 6, and it carries no chlorine, stabilizer, or alkalinity. A heavy downpour dilutes all three at once and can leave you with low pH and low chlorine the next morning.
Runoff brings its own problems: leaves, dirt, grass clippings, and fertilizer that feed algae and use up chlorine. After a big storm, empty the skimmer and pump baskets, skim the surface, run the pump, and test before anyone swims. If the water level rose past the middle of the skimmer opening, lower it so the skimmer can pull debris again.
Adding chemicals safely in the heat
Summer means more chemical additions, and every addition is a chance for a splash or a bad mix. Read and follow the label on every product, and wear gloves and eye protection when you pour or dilute anything, as the CDC advises.
Never mix two products, and never use the same scoop or bucket for two different chemicals. Tablets and cal hypo can react violently with each other, and chlorine mixed with acid releases toxic gas. When diluting acid, add the acid to water, never water to acid. Store chlorine and acid apart, in a shaded, ventilated spot away from children and pets. Heat also speeds up the breakdown of liquid chlorine, so a jug left in the sun loses strength. Keep swimmers out while you add chemicals and until they have circulated.
Heavy swim days and the morning after
A full pool on a hot afternoon can drain your chlorine reserve in hours. The CDC notes that a one minute shower before swimming removes most of the dirt, sweat, and oils that use up pool chemicals, so a rinse rule for guests genuinely helps.
A strong chlorine smell usually does not mean too much chlorine. The CDC says that smell is usually chloramines, irritants that form when chlorine combines with sweat, dirt, and other things that wash off swimmers. Bringing free chlorine back up helps clear them. After a party, test that evening or first thing the next day, bring free chlorine back up, and brush the walls and steps.
Algae season and a testing routine that holds up
Algae rarely appears all at once. It starts as a slick feel on the steps, a dull look to the water, or chlorine that will not hold overnight. Catching those early signs is cheaper and faster than clearing a green pool. This routine covers the hottest months:
- Daily in July and August: free chlorine and pH. The CDC actually suggests twice a day, more with heavy use.
- Weekly: total alkalinity, calcium hardness, and stabilizer. Check salt every few weeks if you have a salt system.
- Weekly: brush walls, steps, and shady corners behind ladders and in swim-outs, where circulation is weakest.
- Weekly: empty skimmer and pump baskets, and clean the filter when its pressure gauge reads about 8 to 10 psi above its clean starting pressure, or whatever your filter's manual specifies.
- During heat waves: run the pump longer. With a variable speed pump, a lower speed for more hours moves the water while using far less energy. ENERGY STAR notes that cutting pump speed in half cuts energy use to about one eighth.
