Disappeared CYA and FC… Ammonia?

[fitchspencer]

Hello TFP. First time poster, long time lurker(learner).

Moved into a house last September, closed pool in October and opened four weeks ago. Pool was slightly green when we opened. I SLAMmed with 10% liquid chlorine and cleared it up in a few days. Chemistry was great for two weeks and water was clear with a CYA of 55 (it was 120+ at the end of last year from the previous owner's use of an inline chlorinator which I no longer use). Two weeks ago we got a solar cover which we have kept on a majority of the time.

Last week, we got carried away with other house projects and did not check on the pool at all that week or remove the solar cover. We opened it up to semi cloudy water and a pH around 6.9. I SLAMmed again with mostly liquid chlorine, but also three pounds of CalHypo the previous owner's had left behind while I was waiting for my liquid chlorine delivery. Pool cleared back up after 4 days.

The past 4 days I have not been able to keep my FC levels up and I tested my CYA and it's at 0. So... What would cause my FC to be disappearing rapidly and my CYA to have vanished?

Pool is currently perfectly clear. 20,000 gallon inground liner pool with sand filter.

My latest test shows:
6/18
0.5ppm FC
0ppm CC
0ppm CYA

 

[Texas Splash]

What would cause my FC to be disappearing rapidly and my CYA to have vanished?

Let's find out. Is your pH still under 7?

By the way, welcome to TFP!

 

[fitchspencer]

Let's find out. Is your pH still under 7?

By the way, welcome to TFP!

pH is currently 7.4. I added 2# of baking soda back on 6/2 before learning that was not the preferred method which only brought it up to 7.1. I have since adjusted my return to point up to aerate which has brought it up to 7.4. My alkalinity is 90ppm.

 

[Texas Splash]

pH is currently 7.4

Good. You had an unusual disappearance of FC and CYA for sure, but I'm not so sure it's ammonia related. Regardless, we might try to get you back on track in a similar fashion. Do you have plenty of liquid chlorine on-hand?

 

[Texas Splash]

With plenty of liquid chlorine ready, here’s what I would recommend:
1. Add nothing to the water yet except for liquid chlorine. Take the FC up to “10” and re-test the water in 10 minutes with good circulation. Don’t wait any longer than 10 min. If the FC crashed below 5, bump it up to 10 and re-test in another 10 min. Repeat this 10-min drill until the FC shows signs of holding between 6-10 ppm after 10 minutes. The closer the FC is to 10 the better, but you get the idea. It needs to show it's starting to hold.
2. Once you see the FC holding, now you can add stabilizer for a CYA target of “30”. Use the sock pre-soaking method. If you aren’t sure about that let us know.
3. With the stabilizer in the water, now you can take the FC up to “12” which will be your official SLAM FC level. Maintain that FC level as best as you can and follow the protocol on the SLAM Process page for best success.

Once you break through that unusual FC loss wall, your FC drops should be less and testing intervals should start to spread out. Then you’re on your way to getting back on track. If you have any questions let us know.

 

[fitchspencer]

Good. You had an unusual disappearance of FC and CYA for sure, but I'm not so sure it's ammonia related. Regardless, we might try to get you back on track in a similar fashion. Do you have plenty of liquid chlorine on-hand?

I read through the pool school section on ammonia before posting and wasn't convinced either. I currently have 4 gallons of 10% on hand. I've gone through 12 gallons in the past week. A few clarification questions:
1. Does it matter if I do this in direct sunlight? What is the sunlight affect on unstabilized chlorine without any CYA?
2. My pool is 16x32 rectangle with only 1 return in the corner on the deep end - 8 feet deep. I have a pretty powerful pump though which provides good circulation. I tend to slowly pour the liquid chlorine in front of return. Where should I be sampling the water from?

 

[fitchspencer]

Last question, can I use granulated trichlor that I already have to bring my CYA back up while maintaining SLAM levels?

 

[Texas Splash]

Does it matter if I do this in direct sunlight? What is the sunlight affect on unstabilized chlorine without any CYA?

This time of day probably not much effect. But if you wanted to wait another hour or two to minimize sun UV competition with the FC that's fine.

Where should I be sampling the water from?

I'd probably grab a sample from around the middle of the pool. Feel free to run a brush around a little too if you like to help move the chlorine around.

 

[Texas Splash]

can I use granulated trichlor that I already have to bring my CYA back up while maintaining SLAM levels?

Later, sure. I wouldn't use it right now during this process though. You need relatively quick/precise additions of chlorine and later CYA. But once you pass all 3 SLAM criteria, sure. You'll probably want more CYA later anyway.