• Calcium nodules
  • Algae on walls
  • Metal stains
  • Excessive chlorine use
  • Greensand filtration
  • Nitrate removal
  • Black algae
  • Calcium buildup on salt systems
  • Fungus under vinyl liner
  • Yellow algae
  • Chlorine demand
  • Chemical toxicity

A paper clip fell into the pool and left some metal stains on the plaster. How can I get them off?

Jack Beane:

There are plenty of products on the market that will quickly remove a metal stain without damaging the finish of the pool.

And there’s a little trick to dealing with rust on plaster: Run the pool’s water through an oxidation reduction process first. That’ll make the stain pop right off when you treat it.

Bob Harper:

Since acids remove fresh metal stains fairly effectively, and the stain is isolated to one specific area, topical treatment should work just fine. One method is to pour dry acid (sodium bisulphate) into a white tube sock. Wearing a rubber glove, place the sock immediately on the stain and gently rub. Another trick is to purchase a length of 1 ½-inch white, PVC pipe. Place the section of pipe over the stain and pour an acid (preferably dry acid) through the pipe and directly onto the stained area. Allow contact for 5 minutes, then brush the area with a pool brush.

Joseph Laurino:

Stains from metals, such as iron, copper, cobalt and manganese, can usually be removed from plaster and fiberglass surfaces using a stain removal product containing ascorbic acid, oxalic acid or citric acid. These materials are known as reducing agents (or anti-oxidants) and they convert the metal stain into its soluble (ionic) form. While these agents often reduce metal staining, they do not remove the metals from the water. Thus, the results are temporary and the stains frequently return.

To prevent the stains from reforming, you must add another product. Currently, there are two options. One is to sequester the metal in the water and limit its reactivity. The second option is to remove the metal from the water chemically using a product designed to accomplish this.

What causes calcium nodules in plaster, and why does it seem to only happen on pools that are done in the winter?

Jack Beane:

Causes of nodules can include de-lamination and shrinkage cracks in the plaster. With some cracks, water can actually flow behind the plaster and back out through a fissure; as it flows back out, it brings salts to the surface with it. These problems can be caused by thin plaster, a wet mix, or other mistakes in surface preparation. It often takes a while to figure out exactly what errors may have been made, and to form a reasonable opinion based on the facts.

An undissolved piece of calcium chloride can also create a discoloration or growth on the surface of the pool, but that isn’t usually the cause. It’s more likely an underlying substrate issue.

Jerry Werner:

The calcium nodules are efflorescence, which is caused by soluble salts in the concrete mix, and activated by water moving through the concrete. These salts are deposited on the surface. As the water evaporates, the salts are left to dry.

During the winter months, calcium chloride is commonly added to heat up the mix, which provides additional soluble calcium salts. The slower evaporation rate in the winter allows more salts to appear on the surface.

This condition can be controlled by using a sealant solution during construction to waterproof the concrete shell; this stops the movement of water through the concrete.

All my chemicals are in balance according to my test strips, but I still have some algae on the walls. Why?

Jack Beane:

It could be an excessive level of cyanuric acid in the water; this makes chlorine less effective, as is demonstrated through an oxidation reduction potential (ORP) test. In addition, it is always prudent to add an algaecide when you have algae. Balanced water does not kill algae.

Bob Harper:

Housekeeping, such as brushing pool walls, is very important in algae prevention. Some algae are free floating and others are surface clinging. This type, on the walls, is of the surface-clinging variety. This is usually remedied by a vigorous brushing of the walls immediately following a chlorine shock treatment and/or algaecide addition.

Circulation also plays a key role in algae prevention. Algae tend to take footholds in “dead zones” which can be in corners, around stairs or, really, any area of the pool. As water moves throughout the pool, it will take the path of least resistance — from return to skimmer or main drain in the quickest, easiest fashion.

Recognizing the dead zones and either eliminating them by angling returns or changing the percentage of skimmer/main drain suction is important in promoting proper circulation.

Joe Sweazy:

There are a couple of reasons why you could have balanced water and still have algae. An excess level of nitrate nitrogen can cause algae growth even in water that seems properly balanced. Test the nitrate level and treat the nitrates accordingly. This may require partial drain and refill.

Additionally, algae growth can occur in areas of the pool with poor circulation where chlorinated water is not able to treat the particular area. This can occur from blockage or obstacles in the pool, or just because of the direction of the return jets.

George Belarski:

The chemical concentrations for water balance may have a lot to do with the overall condition of the water and long-term condition of the pool and equipment, but little to do with the formation of algae. Phosphates and nitrates are not part of the water balance (saturation index).

I seem to be using a lot of chlorine, more than usual. My readings are always low. What do you think my problem is?

Jack Beane:

The answer to this would vary a lot, depending on the situation. Is it a newly started pool? For pools that have just been started up, there’s often a huge chlorine demand.

When you check the water, make sure you’re testing for both free and combined chlorine. Chloramines are already present in city water supplies, and need to be factored into the equation; only when you know the free and combined chlorine counts can you make an accurate determination of how much chlorine is necessary to reach breakpoint.

Bob Harper:

Assuming the test methodology is accurate, it could be a few things. First, I would check for low pH. Chlorine is very pH dependent and in low pH it becomes hyperactive. It forms more of the active form (hypochlorus acid — HOCl) and less of the inactive form (hypochlorite ion — OCl) and will expend itself very quickly.

If pH is in the proper range (7.2-7.8), next check cyanuric acid (a.k.a. CYA, stabilizer or conditioner) levels.

The CYA level needs to be above 30 ppm to enjoy any real benefits from the chemistry. CYA’s sole purpose is to slow the rate at which UV converts chlorine back to its inert form — a chloride. If the amount is below 30 ppm, chlorine levels will dissipate rapidly in direct sunlight.

Joe Sweazy:

You may have nitrates present or high total dissolved solids. High nitrates can greatly increase the chlorine demand. Likewise, high TDS can lead to increased sanitizer demand.

George Belarski:

This is a more common occurrence than most may believe it to be. Most likely there are high chlorine levels rather than low. Typically, the DPD is being bleached out by the chlorine itself, giving a false low reading or analysis.

Most DPD will bleach out to a degree at about 7 ppm, but there may also be some degree of DPD bleaching at even lower levels, resulting in lower readings for chlorine than the actual concentration, unless a form of DPD that does not bleach out is used.

Another method that can be used to determine the correct level of chlorine concentration when bleaching of the DPD is suspected is to do a sample dilution prior to performing the test, and then multiply the result by the dilution factor.

I have a customer that wants to know about a “greensand” filtration system. It’s a type of filtration used on houses to filter manganese or iron out of water. It gets rid of certain funny smells. So the customer wants to know if it’s OK to use on her swimming pool. Are there any industry pros that can confirm or deny the use of this stuff? She wants to use what’s called the Greensand Plus.

Jack Beane:

You wouldn’t want to use greensand as a medium in swimming pool filtration. There are a lot of people who have filled their pools with makeup water that has been run through a greensand filter, but it creates a really tough stain on the finish, especially on fresh plaster. You’ll have to drain the pool to remove it.

It’s better to deal with existing iron in the source water, or just truck in your own water. Not every water source is compatible with a swimming pool. If possible, test the source water, measure the level of contaminants in it, and make a determination about what regular chemical adjustments will be necessary. And on new pools, test the source water before you first put the hose in.

Is there any way to remove nitrates from pool water?

Jack Beane:

First off, if a pool contains 10 ppm of nitrate nitrogen, which is equal to 44 ppm of nitrates, that’s actually a real health issue with pregnant or nursing moms, and with newborns. There’s a syndrome called “blue baby,” where nitrogen ties up the oxygen in the child’s body, leading to circulation problems and other medical issues.

The only solution to a nitrate problem is draining the pool, then refilling it from a fresh water source that’s low in nitrates and nitrites.

Joe Sweazy:

Partial drain and refill is the best approach if nitrates are significantly high (approximately 20 ppm or higher).

However, low levels of nitrate nitrogen can be treated with significant shock dosage. Be aware that nitrates do lead to increased chlorine demand and more than normal will be needed to establish breakpoint chlorination when nitrates are present.

Black algae is the toughest algae to remove. Is it specific to one area of the pool and not another? Does it need sunlight? How low can the chlorine level be before it sprouts? What works and why does it work? Are there downsides to any of the treatments, or is it best to just drain the pool and blast?

Jack Beane:

You’ll rarely see black algae in a well-maintained pool. The higher your pH is, the less hypochlorous acid (the strongest form of chlorine) will be present. A high cyanuric acid level can also inhibit chlorine’s ability to oxidize.

If black algae is a really persistent problem, the best solution is to hire a specialist to get rid of it.

Bob Harper:

While there are numerous treatments based on various chemistries for black algae, the one common thread is the need for vigorous brushing prior to any treatment. Black algae’s primary defense mechanism is a dense, gelatinous membrane that shields it from treatments. This membrane must be breached for any treatment to be effective. However, even when killed, black algae may discolor plaster and aggregate finishes, and require a cleaning to restore the finish.

The real key is to expose the “roots” of the algae. This is accomplished by vigorous brushing. A steel wire algae brush is the best tool for the job (obviously though, not on vinyl liners). Black algae will often grow in areas where it can easily “root.” Rough or etched surfaces are ideal. While it does need sunlight (all plants do), it prefers areas that are not in direct sunlight. It also favors “dead zones” in the pool where circulation (water movement) is poorer.

Richard DeLeo/Dan Miller:

With a white plaster pool surface, a straight trichlor granular poured directly on the algae spot helps eliminate the problem, along with brushing. Follow the manufacturer’s directions carefully.

Other pool surfaces can use a high dose of chlorine shock, and an algaecide especially made for black algae (borate/sodium bromide mix).

Joe Sweazy:

Typically a copper-based algaecide is used, as it has been shown to be most effective on black algae. Of course, using copper can lead to copper staining if too much is present in the water for too long. Therefore, a metal removal agent must be used to remove any remaining copper after the copper algaecide treatment is completed and the algae has been destroyed.

By installing a saltwater system, the calcium on the tile tends to build up much faster than usual. We were told that the answer was to install an acid feeder so as to maintain a constant pH. We have done that on my son’s pool as an experiment. The acid feeder has caused low total alkalinity issues and we constantly have to add a lot of bicarb to counter it. The acid feeder was installed in January just after we had the tile cleaned, and calcium is already appearing again on the tile. Any suggestions?

Jack Beane:

When the acid feeder appears to not be doing its job, it’s time to recalibrate the pH probe. Probes are great, but they’re only as good as their calibration.

Try diluting the acid in the feeder in the reservoir, and using a lower strength in the future. That’ll probably give you a better ability to maintain pH without suppressing alkalinity as much. There’s no way around it, though: You’re still going to suppress some alkalinity.

Also, try adding a product to the pool’s water that inhibits calcium carbonate formation. That may be difficult, though; most stain and scale products on the market are excellent in chlorine systems, but not in salt systems, because salt interferes with their normal sequestering activity. So you’ll have to choose a product that’s proven to work with salt chemistry.

Bob Harper:

In pools on saltwater systems, pH tends to gradually rise. This is because for every one chlorine (an acid) molecule, the electrolytic reaction produces two sodium hydroxide (a base) molecules. As pH rises, the water’s ability to solublize calcium lessens, hence the calcium (scale) build-up.

In areas of lower alkalinity source water, a CO2 injection system would be a better solution than a muriatic acid feed system for controlling this pH rise. CO2 forms carbonic acid, a relatively benign acid. Since it introduces carbonate into the water, it also raises total alkalinity (TA) while controlling pH.

Keep in mind TA is comprised of carbonates, bicarbonates and hydroxides. In areas of higher TA source water, muriatic acid is still the best solution. I don’t recommend the use of dry acids under any circumstances in saltwater pools due to the contribution of sulfate, which can cause forms of scale that will dramatically shorten the chlorine generator’s life.

Jerry Werner:

Other than the mechanical information offered in the question, another possible reason could be a combination of two issues occurring at the same time; that is, as the pool water evaporates, a salt residue may form on the tile, along with efflorescence bleed out from the grout lines.

Calcium in the grout lines that may not be adequately waterproofed is trying to unite chemically with the pool water. This can also be exaggerated by the type of tile that has been used. The more textured the tile, the greater the amount of residue that may cling to the surface.

The winter months have a slower evaporation rate, and moreefflorescence is usually noticed during this time period. Since this is an existing pool structure, it is unknown which waterproofing techniques may have been used to prevent efflorescence. Regrouting the tile to waterproof the new mix may be able to help the situation.

I am having a problem with the fungus that grows under vinyl liners. How can I eliminate it without removing the liner, and what’s the proper way of correcting it when we do replace a liner that has this problem?

Jack Beane:

There are currently no EPA-approved methods for directly eliminating fungus under liners.

Vinyl is porous, so the fungus is actually penetrating the liner; this is why shocking removes it temporarily, but never completely kills it off. The only way to eliminate fungus without replacing the liner is to make sure you’re using clean soil that’s been sterilized with an EPA-approved product or method. Many builders use river sand, which is not clean, and is actually introducing fungus into the soil under the liner.

Bob Harper:

The easiest way is to inject a strong hypochlorite (bleach) solution through the liner into the media (usually sand) behind the liner. After injecting, patch the liner. If replacing the liner, apply a very liberal amount of hypochlorite to the entire affected area. Chlorine effectively kills fungus.

We have been battling sporadic yellow algae in some of our maintenance pools over the years. We have tried polyquats (symptomatic and preventative treatment) and major chlorine shocking (up to 50 ppm, sometimes twice in 2 weeks). We have had it return in a pool with greater than 25 ppm (The chlorine generator was accidentally turned up.) We have cleaned under ladders, treated phosphates, changed circulation patterns, etc. The odd thing is it may be in a pool one year but not the same pools next year.

Jack Beane:

When a pool has recurrent mustard algae, it’s generally accompanied by high cyanuric acid and/or high TDS.

Sodium bromide, which is an EPA-approved method for dealing with mustard algae, is very effective; even more than polyquats. It doesn’t solve the problem, but does a great job of covering it up.

Bob Harper:

Copper-based algaecides are effective in killing yellow algae. Certain specific chemistries that use ‘chloramination’ tend to work well, too.

It is also critical to treat everything that has come in contact with the pool by tossing it into the water during the treatment. This includes vac hoses, vac heads, auto cleaners and pool toys. Even poolside furniture such as lounge chairs. And don’t forget to wash the swimsuits, too. Yellow algae have the ability to go into a dormant state when dry and return to a normal state once wet again. It’s very easy to carry the algae from one pool to the next in cleaning tools.

The common denominator in yellow algae treatments is that you have introduced something into the pool (copper or chloramines) that you typically don’t want in the pool because of the side effects (copper-staining/chloramines-irritants). So, after the treatment, it is important to follow steps for effectively removing these substances to return the pool back to normal operation and eliminate the possibility of side effects.

How can we help consumers understand chlorine demand? Typically, a customer will come into a pool store when they have a problem, get their water tested, then wait for the pool to clear. Other than a lengthy demand test, all chemistry tests treat the pool as a normal situation and do not know there is a possible demand in the water. Once the pool does not clear, the consumer is upset. When replying to their questions regarding how this can happen, any blanket answer you give (lawn fertilizer, etc.) causes them to snap that their lawn is not fertilized, making you feel as if you are trying to cover up a misdiagnosis of the problem.

Jack Beane:

One analogy is that of a porous container: Until the holes are plugged up, you’re going to have to keep adding water, or it’s going to keep draining. Chlorine demand works much the same way: As long as contaminants are getting into the water (which happens constantly), you’re going to have to keep satisfying chlorine demand.

Many factors can affect chlorine demand: Nitrates in the water, high combined chlorine and other chemical balance issues can cause chlorine to work much less effectively.

Bob Harper:

Chlorine demand can come from many sources. Usually, it is an abundance of nitrogen (ammonia) compounds in the water. Some common sources are fertilizer (which can be wind-borne — meaning they might not use fertilizers, but the neighbor does), pesticides (also potentially wind-borne), decaying organic matter (i.e. leaves) and even source water (many municipalities use chloramination to treat water supplies; mine does).

Another source, especially in the eastern half of the U.S., is the practice of coal-burning power plants and factories treating smokestack affluent with ammonium hydroxide.

One of the best proof statements is to print/copy articles such as this one (to demonstrate it is a wide-spread problem) and share it with customers.

It should be noted that there are usually two types of chlorine demand: one, where the pool is still clear, and the other, where the pool is extremely cloudy (turbid). In either pool, a chlorine residual can’t be maintained.

In the clear type, one possibility is to operate the pool normally when it comes to pump run times, housekeeping, water balance, etc., and keep introducing chlorine daily. I would also recommend a twice weekly shock treatment. And, at some time in the future, a chlorine residual will take hold indicating the demand has been met. The pool potentially can be used during the entire treatment time (except during shock treatments). But keep in mind both APSP and CDC standards do call for a minimum of 1.0 ppm of FAC to be present to prevent RWI’s.

With regards to the other type, a partial drain and fresh refill should be considered. Otherwise, ‘nuking’ the pool with up to 50 ppm of chlorine may be the only short-term solution.

I’m getting a lot of questions from customers about toxicity in chemicals. What should I tell them?

Scott Newton

Pool and spa products are evaluated to ensure that they do not present a health risk to swimmers and bathers when used in accordance with label directions. Sanitizers and algaecides are evaluated by the Environmental Protection Agency (EPA). Other pool or spa products are evaluated by the seller.