I'm about a 10 days into my Bacquacil to chlorine conversion. I'm using a K-2006 kit for testing and poolcalculator.com for determining chlorine amounts to add. Initially, obviously, the FC levels were non-existent after adding almost 2 gallons of 10% liquid chlorine twice a day. But on about day 5, the water started to clear and for the last 4 days the water has been sparkling clear and I am only loosing 2-3 ppm of FC over the course of the day while I maintain 15ppm. I've brushed the walls clean of the bacqua fuzz that collected and run my "Aqua Bug" cleaner to keep the bottom and lower sides of the pool very clean. I backwashed LOTS of bacqua crud out of my sand filter over the first several days, but since the water cleared up, the backwashed water looks clean now.

Now the question... My CC stays pretty steady at 3 to 4 ppm despite the amazingly clear water. I'm wondering if the bacqua goop that is stuck in the filter media is what's keeping the CC level elevated. Should I replace the filter sand now, even though my conversion is not "complete" according to the standards from the Pool School (CC <= 0.5ppm)? I seem to be stalled and would love to wrap this up!

Have you passed the OCLT yet except for the CC's (Step 7)?

How much FC are you losing overnight?

I'm losing about 1-2 ppm overnight. I'm wondering if that is only because the baqua that won't backwash out of the filter is consuming the chlorine. I've heard that some of that stuff gets pretty thick in the filter and backwashing won't remove it. If that is the case, why should I try to neutralize that stuff with chlorine over several days when I could just put in my new sand? Is it possible that there is still Baquacil in the water? The water no longer changes color at all when I add chlorine and it is clearer and cleaner than I've seen it since I originally filled the pool three years ago.

If I need to wait until I only loose 1 ppm overnight before changing the sand, I sure will. Is my question about the sludge in the filter consuming the chlorine off base?

Many thanks for all the help here. It's been "fun" watching our pool water go from a completely opaque mustard yellow to diamond clear!

Not at all. That sounds very likely to be the culprit. I would suggest holding shock level for another night with one more overnight FC loss test.

If the results are pretty identical to what you've been getting, I'd go ahead and change the sand but keep your FC at shock value even after the sand change.

Sounds like a plan. Wouldn't you know it... just checked and the last drop of my R-0871 reagent is gone. I ordered another bottle from TFTestKits (Leslie's doesn't carry that reagent). But until that arrives, I'll just have to assume that my usual daily FC loss is still happening and add the corresponding amount of chlorine. You sure go through that R-0871 quickly when you're holding at 15ppm of FC and 4ppm of CC! That takes 38 drops for each test! Thankfully the replacement bottle from TFTestKits is 2 oz. instead of the 0.75 oz. bottle that comes in the K-2006 kit.

Should have some results in a few days. Thanks again for the guidance!

When you know you have a high FC level and you don't need better than 1 ppm accuracy, then you can use a 5 ml sample size where each drop of titrant represents 1 ppm. If you need more visible volume to look at, you can dilute the 5 ml sample with distilled water (for the FAS-DPD test, such dilution doesn't change the number of drops since that depends solely on the absolute chlorine content, not on its concentration).

Sure could have used that tip earlier! I'll have to remember that the next time I'm shocking.

Received my 871 reagent, so I'm back in action...

Was only losing 1.5 ppm overnight, but CC stayed at about 4 ppm. I replaced the sand in my filter and cleaned it out well. The old sand was INFUSED with stinky, yellow baqua stuff. It reeked of CC - that super strong smell that people call "chlorine pool" smell.

I had lost about 5 inches of water due to backwashing and warmer days, so I topped off the water level and ran the filter with the new sand. After a few hours, I tested and both FC and CC were at about 7ppm! I knew the additional water would drop the FC, but does new filter sand cause a CC increase as it gets sanitized?

Got the FC level back up to 15 ppm last night and retested this morning. Only lost 1ppm. CC is still at about 5 ppm. I'm hoping to turn the corner in a day or two. The water definitely smells less of CC after the sand change.

When I put my new sand filter in (with new sand) my CC stayed at 0.

Maybe all of the new water?

Perhaps it is time to check your other levels. This afternoon or evening, it would be a good idea to measure PH, TA, CH, and CYA to see where you are at and to make sure nothing has drifted way out of range.

Good idea. I haven't checked anything but FC and CC for about two weeks. I'll do a full raft of tests tonight.

Just did a full set of tests. Here are the results:

pH: 7.6
TA: 130
CH: 0? (barely, barely turns pale pink when R-0011L reagent added and turns practically clear on the first drop of R-0012)
CYA: <20 (even with tube full to the top, can still easily see the black dot)

Lost 3ppm FC overnight last night 15 -> 12. CC was about 6ppm this morning.
Water is very clear and clean. Temp is about 72 F. Weather has been sunny.

Perhaps I should get the CYA into the 50ppm ballpark?

Your test results are all good, so no clues to what is happening there.

I don't see any harm in adding CYA at this point. It won't help the conversion complete, but it will make it simpler to maintain the FC level. We see conversions that take extra long now and again, but it is always difficult to pinpoint why they take longer.

Thanks for the confirmation, Jason. I'll probably go another couple of days maintaining FC at 15ppm to see if I get over the hump. After that, I might add my CYA to conserve chlorine.

At this point, I think it would have been much, much easier to have drained the pool, replaced the filter sand, and filled it up fresh. I've learned a lot about pool water chemistry, but I don't know if it's been worth the $70+ in chlorine, $18 in extra testing reagents, and two weeks of work!

Overnight: FC dropped 1.5 from 15 to 13.5. CC remains at 6.

I'm really sorry you're getting stuck with CC. Does it smell? Do you know if you started the conversion with a high Baquacil level?

Yeah, it definitely has the CC smell. I don't think the initial Baqua level was high because I hadn't had any Baqua product in the pool for about 4 months - the entire winter.

Tonight I tested in order to top off the chlorine and CC was 2.5ppm. Actually the lowest measurement I've had since I started the conversion. Really hoping the stubborn CC is starting to give up the fight. We'll see in the morning...

This morning, FC dropped from 15 to 11.5 overnight. CC is 4.5. Will this ever end? Could there be Baqua residue in the pipes going to and from my filter that somehow needs to be cleaned out? This thing is making me tired.