Help determining what yellow is on pool liner - possible mustard algae?

We have an inground pool with a vinyl liner. This year we noticed our deep end looked very yellow on the bottom, and the discoloration is now visible along the edges on the pool, and in some cases on the walls. We have spent quite a bit of money at the pool store, having the water tested and trying a variety of chemicals. Yesterday they insisted we add a 60% algaecide but this has made no difference. The water is clear and the edges do not feel any different. Brushing it has no effect. (In researching this problem I thought it might be scaling, but there is nothing to sample- just this discoloration. We were told it was mustard algae but from what I've read, it doesn't seem to match the descriptions.)

My husband is off to the pool store, but I don't think they really know what is going on and just selling us a variety of things to try. If anyone has any suggestions, I would sincerely appreciate them. Thanks!

Can you post a current set of water testing results taken with a drops-based kit?

Also, tell us exactly what all you have put in the pool meaning ingredients and not just product names or 'shock.'

What is the volume, what size pump, what type and size filter do you have?

The pool is an 18x36 inground with an 8 foot deep end, approximately 27,000 gallons my husband believes. The pump is 1hp and has a double cartridge filter. We do not have a heater. We have had the pool for over 15 years and other than black algae a few years ago, never have had a problem. The liner was replaced about 3 years ago.

To open the pool in late May my husband used Surchlor + (sodium hypochlorite 12.5%) and Black Algaecide (dimethylbenzyl ammonium chloride 49.8% , methybenzyl ammonium chloride 2% and 48% other). (The black algaecide was recommended because we had a problem 2 years ago, not for any current problem.) To maintain the pool he used 3" chlorinating tablets (93% trichloro-s triazipetrione, 3.5% zinc sulfate monohydrate, and 2.9% other), chlorine stabilizer (trihydroxy s-triazine), alkalinity plus (sodium hydrogen carbonate) and PH plus (sodium carbonate). He tested regularly using the strips and maintained it within what we thought were the acceptable ranges.

He has had the water tested many times at the pool supply store. Their recommendations were as follows : 1) add Super Mineral Eliminator- 28 oz. (contains citric acid and hydroxyethane diphosphonic acid). 2) add 60% nonfoaming algaecide (poly( oxyethylene dimethylimino) ethylene dichloride 60% other 40%) 3) add 3 1/2 gallons of Surchlor + (ingredients above). Step 3 was done yesterday, so I have given the water results from the pool store from July 4 and today, since the massive addition of chlorine yesterday is not really indicative of what they have been. The results from the 4th are in parenthesis.

free chlorine 14.48 ppm (2.79) total chlorine 14.86 ppm (2.96) combined chorine 0.38 (none given) pH 7.3 (6.9) hardness 16 ppm (19)
alkalinity 52 ppm (53) cyanuric acid 91 ppm (72) copper 0.3 ppm (0.2) iron 0.2 ppm (0.1) borate 0 ppm (0)

The only things that have happened that are unusual is we had over a week of a lot of rain which required draining the pool several times, and the concrete around the pool got power washed. I only mention these things because we are totally stumped. Today, for the first time, I went to the pool store and it was very obvious the owner and workers had a very limited knowledge about this kind of problem and are basically just selling us stuff hoping something works. From the very little I know from reading through these forums, it was obvious they would not be able to help us. I appreciate any advice as to what we should do next. Thank you so much for your help!

Usually yellow 'stains' are mustard algae -- but that's easy to tell. Just brush the spots; if they brush away easily, you have mustard algae.

What you've used:
+ Surchlor + (sodium hypochlorite 12.5%) => bleach. Walmart bleach is 8.25%, and fresher.

+ Black Algaecide (dimethylbenzyl ammonium chloride 49.8% , methybenzyl ammonium chloride 2% and 48% other) => foamy algaecide; should never be used.

+ 3" chlorinating tablets (93% trichloro-s triazipetrione, 3.5% zinc sulfate monohydrate, and 2.9% other) => slightly diluted trichlor

+ chlorine stabilizer (trihydroxy s-triazine) => cyanuric acid or stabilizer

+ alkalinity plus (sodium hydrogen carbonate) => baking soda (AKA sodium bicarbonate, sodium acid carbonate)

+ PH plus (sodium carbonate) => washing soda (AKA soda ash)

+ Super Mineral Eliminator- 28 oz. (contains citric acid and hydroxyethane diphosphonic acid) => citric acid + HEDP. Citric acid should NEVER be used; ascorbic acid (vitamin C) and HEDP, but ONLY if dealing with a specific metal problem. NEVER use it 'just in case'.

+ 60% nonfoaming algaecide (poly( oxyethylene dimethylimino) ethylene dichloride 60% other 40%) => Polyquat; we often recommend it for a variety of purposes (it's an excellent clarifier), but it's not effective against mustard algae.


The really key factor is your CYA level (reported CYA = 91 ppm). That's a test strip reading, so it's not very accurate, but it is likely that your CYA level is above 60 ppm. You'll have to get a K2006 test kit to find out.

Assuming the 'yellow' brushes away, confirming mustard algae, do these things:

=> Read the Best Guess page, linked in my signature, so you can understand what's happening.

=> Order a K2006 kit, so you can find out what your pool levels really are. (test kit page linked in my signature)

=> Purchase an OTO/phenol red test kit (yellow / red drops) locally. Walmart is fine. (Get the 6-way HTH kit if they have it. This is a trimmed version of the Taylor kit, but it has a CYA test.) OTO allows you to test higher values of chlorine, than any other method, except the DPD-FAS method on the K2006.

=> Purchase 12+ gallons of PLAIN Walmart 8.25% bleach, and began dosing nightly with 4 gallons.

=> Test before dosing; once the OTO test result begins to have an orange tint, skip the dose.

=> Brush the pool every other day, AFTER adding chlorine.



. . . Tell us what you find out.

Unfortunately, the yellow does not brush away. I read through many posts in this forum before I joined, and originally thought that might be mustard algae. Since it did not brush away, I then thought it might be scaling and researched that. However, when I got in the pool to try and get a sample, it did not appear to have any roughness. It feels like vinyl. There is no difference in feel between the regular and discolored areas. The color is not a bright yellow, and is in the deep end and along the edges. I don't see anything in the center of the shallow end. We do keep it covered with a solar cover whenever it is not in use.

Any other suggestions are sincerely appreciated. Thanks! Pam

Next test: buy some Vitamin C tablets, and place them on the stain. If it's iron, it will clear in the immediate vicinity of the tabls.

We put 3 vitamin C tablets in various locations in the pool. Each showed a definite change within a few minutes. Thank you so much! What should we do next? (We are still trying to find a pool test kit that was suggested. Unfortunately our local dealers do not have them. We need to order online. )

Once again, thank you for your help. Pam

You have to order the K2006 from Amazon; we've never heard of them being sold locally. (Dealers aren't too excited about a test kit that can 'replace' them!). Click the link in my blue signature block for more info.

You WILL want to get the kit, BEFORE you start cleaning the iron stains in your pool.

The process is -- unfortunately -- somewhat complex. It also tends to be an ongoing issue -- iron is probably STILL coming into your pool, via what ever route it followed before. The most common source of iron, is well water, but it can also be old ductile iron water company distribution pipes. Or, it can be low grade pool salt. Or, something else.

AND, you need to 'immunize' yourself against pool store and pool chemical company sales pitches; NONE of the products named "metal eliminator" or "metal remover", etc actually remove metal from your pool. Instead, they TEMPORARILY re-dissolve it into your pool water. But once the product breaks down, the metal returns to the insoluble form, and stains your pool again!

It's a great scam: you buy "Miracle Pool Metal Magic" and -- lo and behold! -- the metal stain is gone! Wonderful! It worked. You're so happy.

. . . for about two weeks. And the the stains come back. You wait awhile, frustrated and puzzled. And then you go buy some MORE "Miracle Pool Metal Magic", so you can get the 'clear pool high' again.

It's an addiction . . . and the pool store is your dealer.

The ACTUAL process to remove metal from your pool involves multiple steps:

1. Dissolve the metal -- but don't let the algae bloom. (Your chlorine will be zero during this step!)
2. KEEP the metal in the water, while your chlorine is restored.
3. Gradually REMOVE the metal from the water, before the sequestrant (step #2) breaks down.

We can work on that, later.

Right now, you need a test kit. You may also want look into new filter cartridges, and the CuLator product.

=> Filtration is IMPORTANT in step #3. You need high quality cartridges in good condition. If you have OEM cartridges in good condition, you are all set. If you've purchased replacement cartridges from your dealer, and the cartridges didn't have the filter manufacturers logo on them -- you may have a problem. Most of the aftermarket cartridges are low quality Asian made product. Replacing them will help you overall with your pool, but is really important when it comes time to filter out fine metal particles. If you'll post your filter make and model, we can help you find some options.

=> The Culator product will remove dissolved metal from water passing over a bag you place in your skimmer. This is a very gradual process: most of the iron in your pool is NOT dissolved. And it takes a long time for ALL the water in your pool to come into contact with the little bag. There is some good news -- the company that makes these things wants you to replace them monthly. But, as far as we can tell, so long as you keep the bag from getting gummed up, the CuLator pack will last till it's exhausted, which you can recognize from the dark orange color of the adsorbed iron.

One tip: if you get the Culator packs, use them any time you add water to the pool. You can do this by filling via a skimmer that has the pack in it. The more slowly you fill, the better.

Here's some more info:

The CuLator product is a special patented plastic material, contained in a bag that goes into the skimmer, that slowly removes *dissolved* metals from water passing over the skimmer bag. This approach pretty much requires running your filter 24/7, since the bag ONLY removes metals from water that comes into direct contact with the bag. This product can also help if you fill SLOWLY via the skimmer, so the metals are removed from the incoming water. (Not likely to be available locally.)

1 ppm Culator @ Amazon
1.5 ppm Culator @ Amazon
4 ppm Culator @ Amazon
Poolmaster 16242 Poolmaster Skimmer Basket Liner @ Amazon (to protect CuLator from being damaged by scum)