pH Keeps Rising in Your Pool? Why and How to Slow It

The summer we finally turned on the waterfall every afternoon, I thought I had a broken test kit. I would bring the pH down to 7.4 on Saturday. By Wednesday it read 8.0 again, and the tile line started to feel gritty. I bought a new kit, got the same numbers, and kept pouring in acid like it was my job.

It took me most of that season to figure out that the pool was not broken. The waterfall was doing exactly what moving water does. Once I understood why pH climbs, I stopped fighting the symptom and started slowing the cause. Below is the order I follow now, and it uses a lot less acid than my first summer did.

Why pool pH keeps climbing

Rising pH is one of the most common complaints in plaster pools. It is rarely a mystery chemical. In most pools it comes down to carbon dioxide leaving the water, and a few things that speed that up.

Pool water holds dissolved carbon dioxide. The water also holds alkalinity, mostly as bicarbonate. Together they act like a buffer. When carbon dioxide escapes into the air, the balance shifts and pH goes up. Your total alkalinity stays roughly the same while that happens. That is why you can lower pH with acid and watch it creep right back.

Here are the usual drivers, from most to least common in my experience:

• Aeration. Waterfalls, spillover spas, deck jets, bubblers and return jets pointed at the surface all stir the water. Splashing swimmers count too. More surface agitation means carbon dioxide leaves faster.
• High total alkalinity. The more bicarbonate in the water, the more carbon dioxide there is to lose. High alkalinity makes pH rise faster and makes each correction feel temporary.
• New or recently resurfaced plaster. Fresh plaster releases alkaline material as it cures. Pools often show strong upward pH drift for weeks or months after plastering.
• Salt chlorine generators. The cell creates fine bubbles and some byproducts that push pH up. Saltwater pools often need more frequent acid than tablet pools.
• Some chlorine types. Liquid chlorine and cal-hypo have a high pH when added. Much of that effect evens out as the chlorine is used up, but the extra lye in liquid chlorine can add a small upward nudge.
• Fill water. In many desert areas, tap water arrives with high alkalinity and a high pH. Every top-off in the heat adds a little more.

In Arizona I get several of these at once. I have plaster, a waterfall, hot dry wind that pulls water across the surface, and fill water that is hard and alkaline. If your pool feels like it rises faster than your neighbor’s, look at that list and count how many apply.

The fix, in order

Test first, add chemicals second. I write that on the lid of my chemical box because I forget it every spring. The steps below are in the order that saves the most acid and the most frustration.

1. Test pH, total alkalinity and calcium together. Use a liquid drop kit or a reliable photometer if you can. Strips are fine for a quick look, but they are hard to read near the edges of their range. Write the numbers down with the date so you can see how fast pH is moving.
2. Check that your pH reading is trustworthy. Very high chlorine can make some pH reagents read falsely high or purple. If chlorine is well above your normal level, wait for it to drop or use a kit that compensates. Reagents also age, so replace them when the label says.
3. Lower total alkalinity to the low end of its typical range. Many plaster pools with waterfalls or salt cells do well around 60 to 80 ppm, as long as the water stays balanced for scaling. Lower alkalinity means less carbon dioxide to lose, so pH rises more slowly. The common method is adding acid to bring pH down to around 7.0 to 7.2, then aerating to raise pH while alkalinity stays lower. Repeat over several days. Follow the acid label and go slowly.
4. Cut aeration while you lower alkalinity. Wait, that sounds backward. During the alkalinity-lowering process, aeration is your friend. Once alkalinity sits where you want it, reduce aeration for daily life. Run the waterfall for shorter periods. Point return jets slightly down or sideways instead of at the surface. Turn off bubblers when nobody is enjoying them.
5. Add acid in measured, small doses. Use a dosing chart or an online pool calculator with your pool volume. Add the acid to the water, never water to the acid, and pour slowly in front of a return with the pump running. Brush afterward if any lands on the floor. Keep pets and swimmers out until it has mixed.
6. Retest after four to six hours. Let the water circulate before you chase the number. Most of my early mistakes came from adding a second dose before the first one had mixed. If pH is still high, add a smaller second dose and test again.
7. Consider borates as an extra buffer. Borate products can help steady pH and slow drift in some pools. Many owners aim for about 30 to 50 ppm, following the product label. You will need a borate test strip or kit to track the level. Keep borate products away from pets and children, and do not use them if a pet drinks from the pool regularly.
8. Keep a weekly log. Once alkalinity is lower and aeration is controlled, track how many ounces of acid you use each week. If the amount suddenly jumps, something changed. It might be a new water feature schedule, a lot of fresh fill water, or a salt cell setting.

Read nextHow to Close an In-Ground Pool for Winter

Common mistakes that keep pH climbing

I made almost every one of these. Some I made more than once.

• Chasing pH while ignoring alkalinity. If alkalinity stays at 120 ppm or higher, pH will keep drifting up. You will be adding acid forever.
• Adding baking soda by habit. Baking soda raises alkalinity. If your pH is already rising, more alkalinity usually makes it worse. Only add it when testing shows alkalinity is truly low.
• Dumping a big acid dose all at once. Large doses can push pH too low and etch plaster or damage metal parts. Smaller, measured doses are safer.
• Testing right after adding chemicals. You get a reading of a mixing zone, not the pool. Wait for circulation.
• Letting the water feature run all day. My waterfall now runs a couple of hours in the late afternoon. My dog still gets to bark at it, and my acid use dropped noticeably.
• Ignoring calcium and temperature. Low alkalinity works best when the overall water balance is safe. Use a saturation index calculator that includes pH, alkalinity, calcium, temperature and cyanuric acid.

What typical numbers look like

Treat those as starting points. A vinyl pool, a fiberglass pool and a fresh plaster pool may each have different recommendations. Your plaster finisher or equipment manual may also list specific ranges.

A note on new plaster

If your pool was plastered in the last few months, expect pH to rise fast. That is normal during curing. Many finishers provide a start-up plan with specific brushing, acid and testing steps. Follow it closely, because the first weeks shape how the surface looks for years. Do not use the low-alkalinity approach on fresh plaster unless your finisher agrees.

Salt pools need a slightly different rhythm

When my neighbor switched to a salt cell, she called me two weeks later sure that something was wrong. Her pH was climbing almost every day. The cell was working fine. Salt systems simply tend to push pH upward, especially at high output settings.

A few things helped her. She lowered alkalinity toward the bottom of its range. She set the cell to run longer at a lower percentage instead of short bursts at high output. She also started adding small acid doses twice a week instead of one big dose on Sunday. Her water felt more stable within a month. Always check your salt cell manual for its own water balance targets, since manufacturers set them differently.

When to call a pool pro

• pH keeps rising quickly even after alkalinity is low and aeration is reduced.
• You see scale building on tile, heater parts or the salt cell despite your best balancing.
• Your pool was recently plastered and you are unsure about the start-up steps.
• You notice etching, staining or rough spots on the surface.
• You are not comfortable handling acid. A local water lab or service tech can dose it safely.