I'd like for one of the board "gurus" to respond to what I'm about to say.

I've had a situation in which I've shocked the pool. I TOTALLY agree that shock level should be maintained until the pool passes the overnite drop test AND CC level is .5 or less. BUT.........as long as the water is clearing and is just "cloudy"........I don't necessarily see the point in holding shock level values until the pool is totally clear. Especially when the OP has a sand filter like I do which we all know can take a few days to strain all the cloudiness, milkiness, dead algae and such.

Not sure if I count as a guru, but...

The overnight drop test shows nothing living (algae, etc) inpool...
The CC <.5 means nothing chemically in pool...
So, cloudy is usually suspended matter... most often dead algae!
The reason to maintain shock level, is to fully oxidize the dead algae decomp products while it's filtered out
This would theoretically happen at sanitize levels of FC, but maintaining shock assures it.

Fundamentally, we want to describe the rules in a way that is going to be trouble free. Telling people they can stop shocking when they pass the overnight FC loss test has proven to cause too many problems. Those problems don't come up 100% of the time by any means, but they happen often enough that it is worth some extra effort just to be sure you are good.

Questions for you.

At what point do the algae fully oxidize?
Once the algae is fully oxidized then does it realy matter how long it takes to filter out?
If it doesn't, then why waste the chlorine keeping the shock level?
If it does, could you explain why?

I thought dead algae is dead algae.

Hi Jason,

So would it be safe to say that we should pass the overnight FC loss test for atleast two consecutive days to make sure all is good?

The algae itself never fully oxidizes- it filters out and you backwash or clean it out of the filters...
The potential issue, is while it is still in the pool and/or filter, it will start to decompose- the decomp byproducts is what you need to continue to oxidize;
If the pool clears quickly, then this is very minimal; but the longer it takes to filter out, the more decomp will occur, and the more byproducts left in the pool water even after the algae itself is filtered out.
Also, the "waste" of chlorine, with a reasonable amount of CYA, shouldn't be much- most people add Cl every few days anyway, the amount to maintain at shock isn't that much more than normal I don't think ( a function of FC- more FC more loss; but not excessive at our levels I think). You're just maintaining the shock level a few more days instead of letting the FC return to a normal level.

As Jason said, this rule is really in place as assurance the process is complete, and may not be an issue for many people...

If you really do the overnight test correctly, and the result is not at all ambiguous, then you are presumably fine. But we keep running into cases where people are doing the test wrong in one way or another and the results don't mean what they think they mean. If you are doing the test wrong, doing it wrong twice doesn't improve your odds.

"The potential issue, is while it is still in the pool and/or filter, it will start to decompose- the decomp byproducts is what you need to continue to oxidize;
If the pool clears quickly, then this is very minimal; but the longer it takes to filter out, the more decomp will occur, and the more byproducts left in the pool water even after the algae itself is filtered out.
"

Does this mean that the decomposition of the dead algae remaining in the pool untill it filters out or is backwashed out actually creates a "CC" situation? Even after the pool passes the overnite test and shows ZERO CC when tested properly? I mean, can the CC cycle start right back up again with the dead algae remaining?

Exactly- no CC since no decomp byproducts, but once decomp starts the decomp process will release chemicals previously "locked" into the algae cell structure- and the FC will oxidize this into CC...

Good question?

This is also why we encourage everyone to scoop out as much solid material (leaves and detritus) as possible- it removes potential clogs and mechanical issues, and it removes the decay products from the pool before the decay happens!

Thanks madwil: Just another question and response added to my "notebook"! Dang that thing is gettin' thick.

"The potential issue, is while it is still in the pool and/or filter, it will start to decompose- the decomp byproducts is what you need to continue to oxidize;
If the pool clears quickly, then this is very minimal; but the longer it takes to filter out, the more decomp will occur, and the more byproducts left in the pool water even after the algae itself is filtered out."

will DE speed this process up?

DE usually provides finer filtration so would remove physical material from the pool faster assuming you backwash/clean the filter as appropriate. For those with sand filters, adding some DE to them can speed up the clearing if it's slow due to too many particles that aren't getting caught in the filter. Generally speaking, having more physical removal of larger pieces and then better circulation (including brushing) and better filtration all result in getting organic material out of the pool faster. The net result is that it takes less chlorine to take care of whatever is left over.

So the point brought up by this thread, as I understand it, is that while initially you want to physically remove leaves and other large pieces of organic material and then shock the pool to kill any growing algae, that once dead it's more important to physically remove it through circulation/filtration/backwash/clean than to continue shocking. I would say that is true and would probably use less chlorine, but whatever is leftover will get oxidized faster at shock level than at normal levels. So if after getting the pool visibly clear one continues to measure CC or one has too high an overnight FC loss, then getting back to shock level is usually a good idea.

So while a modification of the basic procedure would be to stop shocking during the "clearing dead algae" phase and then once the pool is clear to shock again if the CC is high or overnight FC drop is high, this is more complicated to explain. It is also something we'd have to see working in practice and not just in theory.