Algea keeps coming back

[tomvalois]

A few weeks ago, I came home from a trip to a pool full of algae. I slammed it according to the TFP method, which seemed to work. However, about a week ago, the algae came back. I slammed it again, this time being much more careful and frequent with my chlorine measurements. Three nights ago, it passed to the overnight free chlorine loss test, the water was clear and there was no measurable combined chlorine. This morning, there was algae on the steps and some of the sides. I tested the water, and the chlorine hadn't even dropped to its target range from the slamming. Here were the numbers from the test:

CYA: 50
Calcium Hardness: 210
Total Alkalinity: 90
Free Chlorine: 11.5
Combined Chlorine: 0.0
pH: 7.6
Water Temperature: 87.3 degrees

Can anyone venture a guess as to how the algae is forming? Could it be mustard algae, or some other resistant species? If so, how do I determine what kind of algae is it?

 

[Texas Splash]

It would be odd (perhaps not impossible) but odd for algae to survive under such a strong chlorine/CYA ratio. Are you sure it's algae? Are you able to scrape or secure any of it onto some paper and smear it to see if it's green?

Next question might be circulation. Is the water moving well around the steps and sides? Speaking of steps and sides where you saw this stull, if it in the shade or exposed to the sun in those areas?

These are some things that may help give us a better picture of your situation.

 

[tomvalois]

I am not sure it is algae, but it looks like algae. Are there other microorganisms that can cling to pool surfaces the way algae does? It isn't something just settling in the pool (like pollen, for example), because it clings to the sides. There is pretty good circulation throughout the pool, including the steps. The stairs (and for that matter most of the pool) are exposed to the sun for several hours a day.

 

[Texas Splash]

Because those areas receive ample sunlight each day, I wouldn't think it would be mustard algae. But if the situation persists and do you do SLAM again, you could raise the FC, after passing the normal SLAM, to MA levels for 24 hours. Even though you passed the OCLT before, is there any chance you still have some growth hiding somewhere like behind the pool light, in a ladder/hollow steps, or something like that perhaps? Do you have a pool cover that may have remnants of algae underneath perhaps? Just throwing some ideas out there for you.

 

[tomvalois]

I don't have a pool cover, and I did a thorough job of brushing, but I hadn't considered the pool light. There is some discoloration on the inside of the light cover. I won't be able to tell whether it is algae until I get it apart. I'll disassemble it, clean it, SLAM again, and then raise the FC to the MA level for 24 hours just in case it's mustard algae. Thanks for the responses. If that doesn't fix it, I'll be back....

 

[tim5055]

Are you sure about your CYA? Exactly one month ago in a different thread you listed your CYA as 90. To now have 50 you would have had to replace almost half the water in your pool. My gut is telling me you have been under chlorinating because of high CYA.

 

[tomvalois]

That 90 CYA reading from a month ago was not right. I was stopping the test when the dot started to disappear, not when it had completely disappeared. Doing the test correctly showed that it was between 70 and 80. And shortly after that post (after reading about how difficult it can be to maintain a pool with high CYA levels), I started draining off a little water each day and letting the afternoon thunderstorms dilute the pool. Right before I slammed it the last time, I finally gave in and replaced about a third of the water. I figured it was better to pay a few extra bucks on my water bill for one month than to be stuck forever paying for twice the chlorine I should be paying for. I have tested the CYA several times today, and it has been right around 50 each time. After reading your post, I even had my wife do the test to make sure she saw the dot disappear at the same point I did.