Ask John, Part 1 Real Pool Chemistry Questions, Real Answers

Last week I closed the show by asking you to send me your questions. Anything. Your pool problems, your arguments with your pool guy, the stuff that doesn't make sense no matter how many times you read the back of the bottle. I did not expect what happened. By Thursday, the Ask John page had more traffic than it had since launch. Emails, Facebook messages, DMs on Instagram and Reddit, which is interesting because I haven't posted on Reddit in several weeks. People went and found me anyway. I read every single one of them. For 10 episodes, this show has gone in one direction. Me talking, you listening. Today, that changes.

Are you ready? Let's get into it. Welcome to the Pools Scientific Podcast, elevated pool care through science and technology. I'm John Cooper, and whether you own a pool, service them, or run a pool company, this show is built for you. Real Chemistry, Real Answers. Every Tuesday.

If I'm honest, those first ten episodes were really about one question. Could you trust the chemistry we're talking about? Can you trust the guy explaining it to you? I think by now we've earned that, and at least with the people still listening. The show itself isn't changing. What's changing is where it points. Starting today, your pools and your actual questions are the subject, not just for this episode. I'll weave them into the podcast going forward. We have five real questions from five real pools today. But first, I want to clear out one of the questions I get asked more than any other in the field, on Reddit, in my own inbox.

It comes up so often it deserves to go first before we even get to the other five questions. How long should you run your pump? Every pool owner has heard a number for this. Eight hours. It's on the laminated card at the pool store. It's in half the owner's manuals that ship with the equipment. Eight hours. It's like scripture. Here's where that number actually comes from. It's called turnover rate. Turnover rate is the time it takes your filtration system to move the entire volume of your pool through the filter once, based on your pump's flow rate and your pump's volume. Equipment manufacturers calculate it so they can size a pump and filter correctly for a given pool.

That's it. That's the whole purpose of the number. It's a sizing calculation for the people that build the equipment. Somewhere along the way, eight hours covers your turnover, quietly turned into run your pump for eight hours and you're done. Nobody's decided that on purpose. It's just what happens when a sizing spec drifts far enough from the engineers who rode it. Here's what the turnover rate doesn't tell you. It doesn't tell you how long your sanitizer needs to actually contact the water. It doesn't tell you how long suspended particles need to pass through the filter media before they're actually caught, which usually takes several passes, not just one.

And it says nothing about the dead corner of your shallow end that barely sees flow even when the pump is running, because turnover rate assumes perfectly mixed water and your pool is not a beaker in a lab. So what? What do you actually do with that? You run the pump twenty four hours a day. Every day. Not just because the chart says so, because continuous circulation is the only way to keep sanitizer in contact with every part of the pool. All the time, instead of in eight hour bursts followed by 16 hours where nothing is moving and nothing is being treated. Algae doesn't wait for your filtration schedule, neither does the chlorine demand from yesterday's bather load, or a leaf that fell in at midnight.

A body of water that filters and circulates is a pool. One that doesn't is a pond. If it's the cost that worries you, it shouldn't. A variable speed pump runs continuously on a low setting, and it costs less to operate than a single speed pump running hard for eight hours straight. And it does a better job because slow and constant beats fast and intermittent for nearly everything chemistry touches. That's the move. Take the rule everybody quotes, ask what it actually means for your specific pool, and act on that instead of the number on a printed card. I'm going to be doing more of these going forward.

I'm calling it so what? Conventional wisdom says one thing. So what do you actually do with that? You'll hear it again. Now let's get to the questions. First one is an email. I got it from Mark Yancey, who has a residential pool with a salt chlorine generator. Mark's question is Does the screen on my pool cage reduce the UV that degrades my chlorine? That is a really good question, and the answer is yes, but with a number attached. A typical pool cage screen blocks somewhere between 30 and 50% of incoming UV. That's real. Chlorine loss without CYA protection runs roughly 1 to 2% per hour from UV degradation alone.

So cutting the UV hitting the water meaningfully slows that loss. Here's the part that matters more than the percentage though. It doesn't change what you do. You're still managing free chlorine relative to CYA, the same 7.5% rule either way. The screen might mean you lose less chlorine when the sun is out. It might mean your CYA can run a little lower than an uncovered pool and still hold its sanitizer level. But you don't manage to the screen. You manage to the test kit. Let the data tell you what the screen is actually doing in your specific pool, because cage construction, screen type, sun angle, and time of year all change the real world number more than any spec sheet will.

Thanks for the question, Mark. That was a good one. The next one came from the swimming pool subreddit from a user named Barnacle Meat. Mr. Meat has a salt chlorine generator, and he wants to know what CYA level I recommend for a salt water pool and why. Based on science and experience, I recommend 30 to 50 parts per million CYA. My hard ceiling is at 50, no exceptions, salt pool or not. I know that ceiling runs lower than what you'll see on Trouble Free Pool or most of the calculator tools out there, especially for salt pools where a lot of that community will tell you 70 to 80 is fine, even up to 100.

I want to be fair to that position before I tell you why I don't follow it, because the chemistry behind it is real. Most people think CYA's only job is blocking UV so sunlight doesn't burn off your chlorine. That's true, but it's not the part that actually matters most. The bigger thing CYA controls is how much of your chlorine is even available to sanitize in the first place. The actual sanitizer in your water is hypochlorous acid, not the free chlorine number that sits on your test kit. CYA binds chlorine and holds it in reserve, and the equilibrium between bound and active chlorine, the research Richard Falk's work is built on, going back to a 1974 paper by O'Brien, Morris, and Butler, shows that what actually matters is the ratio of free chlorine to CYA, not either number by itself.

Hold that ratio steady and the hypochlorous acid level stays roughly constant whether your CYA is at 30 or 100. UV protection is secondary. Sanitizing power is the primary concern and CYA governs it directly. That's the real basis for the 7.5% rule, and it's also the basis for letting CYA run higher in a salt pool since the chemistry itself doesn't fall apart above 50. So why do I cap it there anyway? I have two reasons, and neither one is chemistry. Both, however, are a reality. First, holding that ratio gets harder, not easier, as CYA climbs. At 50 parts per million CYA, you need 3.75 parts per million free chlorine just to stay even.

At 100, you need 7.5. At 150, 11. Every step up means more chlorine has to go in, more often with less room for error, and an ordinary pool owner managing this without a chemist on call is now fighting a moving target that gets away from people fast. Second, the equipment most people actually have can't deliver what the ratio demands at high CYA. Salt cells, chlorine feeders, ORP-based dosing systems, tablet floaters, none of them are built to push and hold 5, 8, 12, 15 parts per million chlorine day after day. Getting there at 100 plus parts per million CYA means constant heavy dosing with calhypo or liquid chlorine by hand all the time.

That's not a backyard pool routine. That's a part-time job. So 50 isn't where the chemistry breaks, it's where the math stops being something a normal pool with normal equipment and a normal owner can actually keep up with safely. Past that point, the ratio is still technically correct. It's just not realistic. Now, specifically for salt pools. CYA in a salt pool is a double-edged variable in a way it isn't in a non-salt pool. Your salt cell is making chlorine continuously, all day, every day. That continuous production means you don't need as large a CYA buffer to protect a big single dose the way you would with tablets or liquid going in once or twice a week.

So you can run the lower half of that 30 to 50 range and do it comfortably. Here is something, however, to watch out for. If you're using trichlor tablets in a salt pool, whether they're in a floater in your skimmer basket or anywhere else, they're adding CYA on top of what's already there. CYA in a salt pool doesn't go anywhere on its own, it only goes up. I've seen salt pools blow past fifty, eighty, a hundred without realizing it because they kept a tablet in the skimmer or in a floater, and it was just a habit before they had the cell installed. Keep an eye on that CYA.

The next question is the most technical question I think I've received, and it's worth slowing down for because it's a discrepancy a lot of salt pool owners are going to run into. Alec Wasserman wrote in. He has a residential salt chlorine generator pool, and he's in South Florida. His numbers are all in range and look good, but he's getting two different LSI readings from two different calculators. The Taylor calculator in his test kit shows his LSI at minus 0.13. The pool scientific calculator shows minus 0.41. Which one should he trust and how does he correct it? Simply, you trust mine. And I want to walk you through exactly why, because the why is actually the lesson here.

The discrepancy between the two calculators is TDS, total dissolved solids. A salt chlorine generator pool sitting at 3,000 parts per million salinity has dramatically elevated TDS compared to a standard non-salt chlorine pool. And TDS is a real input to the Langular Saturation Index equation. It affects the ionic strength of the water, which affects how the equation behaves. The Taylor water balance calculator doesn't ask you for an actual TDS measurement. It assumes a standard background level and runs the math from there. My calculator asks for it, because in a salt pool, ignoring it isn't a small rounding error. It's the difference between negative 0.13, which reads as mildly aggressive, and negative 0.41, which reads as meaningfully aggressive and headed towards problems with metal components and grout lines if it sits there.

Negative 0.41 is the real number for Alex Pool. Another question from the swimming pool subreddit. This one from a user named Striking Manor 9380. It's a fiberglass pool with white powder on the side walls. It brushes off clean, but it comes back in a day or two. His question is the gel coat failing? Would a CSI or LSI reading help? First of all, it's not the gel coat. I want to put that worry down immediately because gel coat failure is a thing and it's a different animal entirely, and it doesn't look anything like this. What you're describing is calcium carbonate precipitation scale. Your water is sitting on the positive side of the LSI and it's depositing dissolved calcium back out of solution onto the surface, which is exactly the white powder you're brushing off, and exactly why it keeps coming back, because the water is still positive after you brush it away.

For a viberglass pool specifically, your calcium hardness target should sit at 100 to 150 parts per million. That's noticeably lower than a plaster pool. If your hardness has crept above that target, that's very likely your driver. Pull it down towards 150, walk your pH back towards 7.4, and run the actual LSI calculation rather than guessing, and then adjust from there. I'm not telling you this works just in theory. I've got a fiberglass pool right now in what I call the lab, one of the active pools I manage across the Kansas City Metro. The same pools I treat as a running database for everything on this show.

Salt system, same basic picture with visible calcium deposits on the surface. Over the course of about six weeks, we pulled the alkalinity down, used aeration to keep the pH in check, and to push the LSI to about minus 0.20, and let the water actually redissolve what it had already deposited instead of just stopping new scale from forming. 99% of those deposits are now gone through chemistry alone. No acid washing the surface, no draining the pool, no renovation. The water did the work once we gave it the right LSI to do it in. The fiberglass is as smooth as when it was new. Total chemical cost for that whole correction was two gallons of myriatic acid, about eight bucks.

The alternative, if that scale had kept building, was a fiberglass refinish that starts at about$10,000 in this market. $10,000. $8 instead of$10,000. Keep that number in your head because we're coming back to it. The last question came from the pool subreddit from a user named WankerTank. That's kind of my favorite name in all of Reddit, I think. He has a residential pool, and here's the detail that makes the question interesting. The neighbor's dogs drink out of it.

He wants to know what the actual trade-off is for using borates, and is it safe? I like this question because it's a real-world constraint most chemistry advice ignores completely. Bora toxicity is real. I'm not going to pretend otherwise. Toxicologists measure that kind of thing with a term called the LD50, short for lethal dose 50, the dose at which 50% of a tested population would die. The LD50 for borax in dogs is quite high. A dog would need to consume a volume of pool water that simply isn't realistic from casual drinking, even daily casual drinking, to approach that dose. The practical concern at normal pool concentrations isn't poisoning, it's mild gastrointestinal upset if a dog drinks a genuinely large amount in one sitting.

The kind of thing that resolves on its own with what's known in the pool industry as a fecal incident. Maintain your borates at 50 parts per million, which is the effective range for the pH buffering benefit you're actually after. The neighbor's dogs are fine. I'm doing a full episode on borates, supporting chemicals, scale inhibitors, the whole ancillary cast, and borates will get the deep treatment there. So there you go. Five questions, five real pools. That's what this current part of the podcast is about. Next, I want to tell you about something I've been building quietly for a few weeks now, and I want to tell it to you through an actual experience with a person named Warren Bell, because that's how it happened.

Here's the problem. Pool store visit runs you somewhere between$100 and$200 every visit. And by the industry's own informal math, about half of what you bring home from the visit creates a new problem that requires yet another product to fix it. That's not me being cynical. That's the pool store business model. The cycle doesn't stop because stopping it doesn't make anyone money. Here's the bigger number underneath that one. A replaster job starts at$15,000 to$30,000. A vinyl liner is three to five thousand dollars at minimum. Fiberglass refinishing starts around ten thousand. Correct chemistry, managed consistently, prevents essentially all of that. Remember that fiberglass pool from a few minutes ago, the one with the scale?

Two gallons of myriatic acid, about eight dollars total. That's what it costs to prevent a refinish that starts at ten grand. That's not a hypothetical I made up to sell you something, that's one real pool this year in my own lab. It's real chemistry, real savings, and a real answer waiting for you before your pool gets worse instead of after. A single bottle of algecide your pool never needed in the first place is an expense. This is the best return on investment your pool will ever see. The pool store sells you a cycle. I built the chemist circle to end it. And here's where Warren comes in.

Warren Bell is sixty-three and a retired television writer. He found this podcast on Spotify. He's here in Kansas City but outside our service area by a bit. He was referred to me by one of our pool builder partners. Warren texted me at the end of May with a water analysis from a pool store. His pool scored forty percent. Forty.

I asked him to send me the actual numbers, not just the store's grade, because a score of one out of a hundred tells you nothing about why. So he sent them over. CYA at 78, free chlorine basically nonexistent. TDS through the roof at 5,000. He has a plaster pool with a salt chlorine generator, and it's about 18,500 gallons. And his pool had an active infestation of whitewater mold biofilm growing in the plumbing and on the surfaces, even on the bottom of his autocover. I told him exactly what I tell anyone with a biofilm problem. No shortcuts. Here's the plan I gave him. Partial drain and refill cycles to bring that CYA down to something workable.

His TDS as well. Liquid chlorine in real volume to shock the whole system and hold it there long enough for the biofilm to actually die, not just look better for a week and then come back. Warren did the work. He drained, he refilled, he dosed. And every step of the way he texted me his numbers, and I checked every single one before he moved to the next step. It took nine days and about twenty gallons of liquid chlorine total. On day nine, Warren sent me a photo of the water clear enough to read a dime off the bottom. He went swimming that weekend.

Warren left a five-star review on Apple Podcasts. You can read it there. He called me, and I'm quoting him directly here, the Neil deGrasse Tyson of Pool Science. I told him I thought I was considerably more handsome than Neil deGrasse Tyson, at least in the dark. Warren said he meant every word anyway. His eleven-year-old son is apparently very impressed. And I will take that win over the review any day. Warren and I did all of that by text, because at the time I didn't have anywhere to put what I was building yet. Warren was the proof of concept this whole thing needed. I owe him for that, so thank you, Warren.

And that's not where it ended either. Warren still texts me. Weeks after the crisis, long after the biofilm was gone and there's nothing left to fix. He recently ran a superchlorination pass on his own, just to be sure, and the water's been clear ever since. He also let me know that he opened his filter that used to be caked up with slime and found nothing but ordinary dirt. His CYA is down to 19. He caught his alkalinity creeping up past 100 on his own and brought it back down himself with some muriatic acid. Right now, he's bringing his calcium hardness level up in stages with calcium chloride so he can finally trust his LSI number instead of just one skewed by hardness that's been low since this whole thing started.

He's even started running his salt cell harder during the day when the covers open and easing it back at night. That's not a man waiting on a pool store to tell him or sell him something. That's a man running his own water. He sent me one more thing recently, I haven't Able to shake. He said his pool water is, metaphorically speaking, like homemade pasta sauce versus the stuff in the jar. Always better because you can taste the effort and the care that went into it. He said he doesn't think he's ever actually enjoyed the feel of his own water until now. I get rewarded in plenty of ways for doing this show.

That text might be the best one. And now I'm hungry. It's called the Chemist's Circle, and you can join right now. I want to tell you why I built it because why matters more than what's actually inside it. Every pool owner I've ever met has been treated the same way by parts of this industry. Like you're not smart enough to understand your own water. Like the only thing you need to know is which bottle or bucket or whatever to buy next. You spent real money on that pool, sometimes more money than you spent on your car. And somewhere along the way, too much of this industry decided you didn't deserve to actually understand what you own.

That's not an accident. Confusion sells product. A clear answer doesn't. I work hard for what I earn, and I know you do too. I also know that if a system is rotten at the core, it'll show you at every intersection, and this one has been showing you for decades. The chemist circle is the exact opposite of that. On purpose. It's a real person who happens to be a chemist, looking at your real numbers and telling you exactly what your pool needs and exactly how to fix it. No product to sell you, no guessing, no being talked down to, just the actual answer based on real science with easy step-by-step instructions, the same way I'd give it to you standing at your own equipment pad, just like I did it with Warren.

There are a few ways into it, and what's included with each one, along with what it costs, is on the website, not buried in this episode. I'd rather you read it clearly so you can go to poolsscientific.com slash chemistcircle and it's all there. There's a link there at the website for you to take a look. One thing I do want to say plainly with a smile on my face. However you join, the way you reach me is through that structured submission, not my personal cell number. No, you don't get to blow up my phone at midnight because your pool turned green 20 minutes before the party starts.

That text will still be there in the morning and so will I, with a real answer. Keeping it to the submission is the only way I can give everyone in the circle that same fast real answer instead of just whoever has my number and isn't shy about using it at 2 in the morning. I built this because the thousands of pools I've managed over my career have convinced me that the chemistry I use is right, and Warren convinced me it works just as well for someone I've never met over a text thread. The circle is that same conversation. It's just open to you now, too. Check it out at poolsscientific.com.

Next Tuesday, we go back to straight science. Combined chlorine and chloramines. The stuff that's actually making your eyes burn at the public pool and is not what you think it is. And it has almost nothing to do with how much chlorine is in the water. Keep the questions coming. The Ask John page never closes. Some of you are going to end up on air again before this stretch of episodes is through. And if today's episode is the reason you finally want a real answer for your own pool, the Kim's Circle is open. poolsscientific.com. Check it out there. Bring me your numbers.

The Pools Scientific Podcast is an independent production by John Cooper and Pools Scientific LLC. Find us at Pools Scientific.com. If this show has been useful to you, share it with someone who takes their pool seriously. New episodes every Tuesday.