Green pool but only .2 CC -- is that possible?

singerteacher · Jul 27, 2012

Long story short: Just after vacuuming up a bunch of pollen, I blew an electrical switch and had no pool pump for 4 days until I could get someone to fix it. (I don't mess with electricity and water.) My pool turned green, so I brought it up to mustard algae level for 24 hours (just in case). I did this by assuming FC was 0 and adding 12.5% chlorine according to ThePoolCalculator, once in the morning, then again at night. (No, I did not test at night, but I added the same amount both times.)

This afternoon, I let the FC drop to regular shock level. I tested the water, and it was exactly at shock level (15) ... great! But what threw me is that I had just 0.2 for my combined chlorine! I expected it to be higher, so I waited an hour and tested again -- exact same results (15 FC, 0.2 CC).

Have I actually killed all the algae, even though my pool is still green (getting better slowly)? Or is my reagent bad?

mpacheco72 · Jul 27, 2012

You still need to shock if your water isn't clear.

257WbyMag · Jul 27, 2012

It is possible to have 0.2 CC with green water. Continue shocking until your water is clear, CC is 0.5 or less, and you lose no more than 1 ppm of FC overnight.

singerteacher · Jul 28, 2012

Six hours later, and FC is still 15, but now CC is 0. Don't worry, I will keep shocking until it's not green. Just added 2 more gallons of shock.

What confuses me is: why is my CC at 0? I have always thought that CC was an indicator of whether the free chlorine is "combining" with anything -- in other words, killing something. In my current case, I would think that 0 CC means there is no algae left to kill. It is all dead, or else I would have more than 0 CC.

If it's all dead, why do I need to keep shocking? Shouldn't I be able to maintain at 5 FC now? I know that's a bad idea, but I don't know WHY.

Another example: When I have yellow powder in the divets of my pool steps, I always wonder whether it's pollen or mustard algae. If I test my water and my FC is the 3 -5 range, and my CC is 0, I automatically think it's not algae, because if it were, I would have some combined chlorine.

But now I'm thinking that 0 CC doesn't mean that. So, what exactly does it mean?

dattia · Jul 28, 2012

Having 0 cc is a good sign, but there are three criteria to complete the shock process. When taking my pool from a swamp to an oasis took more than a month. I wasn't showing any CC after the first few days, but the appearance of my pool told me I wasn't finished. If your pool is still green, your process is not complete and you are not ready to maintain at 5FC... Maintenance starts when you pass the overnight test, CC is less than 1 AND your water is clear. Perhaps your organics are dead, but your process is not finished.

singerteacher · Jul 28, 2012

Yes, don't worry everyone, I'm not going to stop shocking.

"Perhaps your organics are dead, but your process is not finished." Can anyone give me more details about exactly what process is still going on? I am not trying to debate whether I should keep shocking -- I'm trying to understand why. So far, everyone just keeps telling me to keep shocking. Yes. I know that. And I hear you repeating it to me. And repeating it does not help me to understand WHY. Does 0 CC actually mean "there is no living algae"? Or does it mean something else?

I think I need a chemist to explain it to me.

chem geek · Jul 28, 2012

0 ppm CC just means that there isn't chlorine combined with ammonia or some organics. If the pool is still not clear, then there is dead algae or other particulate matter that needs to be cleared and the best way to do that is via filtration. The chlorine at shock level just keeps any algae from clearing and helps to oxidize some chemicals, but it won't oxidize everything so you need filtration to remove the rest (the chlorine will usually turn the green algae cloudy white, but it still needs to be physically removed in the filter). So 0 ppm CC and clear water are two criteria. The third is not losing significant chlorine in the overnight chlorine loss test. That is because you can still have nascent algae growth or mostly dissolved chemicals that chlorine can still react with and those need to be fully oxidized or removed. So when you have little overnight chlorine loss (<= 1 ppm) and the water is clear and <= 0.5 ppm CC, then your pool is in good shape.

If your pool water is now a clear green (i.e. not cloudy), then you may have metals in the water and that requires a different approach of adding a metal sequestrant, but you would lower your chlorine level and usually pH first before doing that.