- 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?
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.
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.
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
What causes calcium nodules in plaster, and why does it seem to only happen on pools
that are done in the winter?
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
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?
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.
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.
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.
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
I seem to be using a lot of chlorine, more than usual. My readings are always low. What
do you think my problem is?
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
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.
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.
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
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.
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?
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
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?
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.
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
Other pool surfaces can use a high dose of chlorine shock, and an
algaecide especially made for black algae (borate/sodium bromide mix).
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?
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.
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.
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?
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.
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.
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.
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.
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
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.
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
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?
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.