Shock or Stabilizer 1st

[Bobbyk]

I have a SW IG pool that needs both Shock and Stabilizer (which is just about 1/3 low). Does it matter which one I add first?

 

[chem geek]

If you add the stabilizer first, then the chlorine shock you add will not break down quite as quickly in sunlight, but will also be less effective. So it really depends on why you are shocking in the first place. You normally don't need to shock a pool unless you have some issue you are dealing with such as algae growth or measurable Combined Chlorine (CC) or some "accident" (fecal, urine, dead animal, etc.).

Richard

 

[Bobbyk]

I am shocking because I have about 3.4 ppm of Combined Chlorine, and .2 FC. In this case, should I shock or stablize 1st?

 

[chem geek]

Yikes! I'd shock it first, stabilize later (since you already have some stabilizer -- it's just 1/3rd low) when you get the CC below 0.5 ppm (use the 25 ml sample for testing so that you get 0.2 ppm resolution).

This is a very unusual situation to have such high CC and low FC. What happened? This sounds like ammonia may have gotten into the pool somehow.

 

[Bobbyk]

I don't know what happened. For 10 years, using Chlorine, I had no problems. Last year, I had a bad algea problem, and switched to SW at the end of the season. This year, I have not been able to get my Chlorine right yet. Someone suggested my Phosphates might be high, and depleting it. I tested and am at over 1,000 ppb. I ordered a Phosphate eliminator, which hasn't arrived yet. I plan to add that 1st. After my Phosphates are eliminated, then I will take your advice and shock, then stabilize.

 

[chem geek]

Yes, high phosphate levels can have algae grow faster and can get to a point where your SWG can no longer keep up, especially if you don't maintain an FC level that is 5% of the CYA level. You can shock the pool to kill the algae and maintain higher FC levels to keep the algae from coming back, but using a phosphate remover is another approach, though more expensive. I have nearly 3000 ppb phosphates in my own pool yet maintain it with chlorine alone, but if I let the FC get below around 5% of the CYA level the water starts to turn dull from algae growth (since my pool is manually dosed, I target an FC that is around 10% of the CYA level to be on the safe side).

When you say SW for saltwater, do you mean you now have a saltwater chlorine generator (SWG) that automatically generates chlorine? Were you maintaining an FC level that was 5% (or more) of the CYA level when this problem occurred? When you shock at the levels recommended in this table (you may need to look at the non-SWG table if your CYA is low), keep the FC at shock level until the FC doesn't drop by more than 1 ppm overnight and you measure < 0.5 ppm CC and the water is crystal clear. Then, you can increase the stabilizer level and be sure to increase your FC accordingly to be at least 5% of that level (or follow the chart I linked to). You can then see if the chlorine demand is normal and if the CC doesn't increase. You may not need to use the phosphate remover at that point (i.e. the shocking and maintained FC level may be all that is needed).

Richard

 

[Bobbyk]

That's right. I now have a now have a saltwater chlorine generator (SWG). Like I said, I just installed it at the end of the season last year, and I haven't gotten it to maintain the FC level yet. My test for Phosphates only goes up to 1,000 ppb, so it may be anything above that. I sent away for 6 liters of Phosphate Remover, and I think it was about $60. If it works, it will have been well worth it.

Should my goal be to keep my FC at 5% of my CYA level? Then if I shoot for 2 ppm FC, I should shoot for 40 ppm CYA, right?

 

[chem geek]

Follow this chart for chlorine levels. If your CYA is lower, use the non-SWG table for now. The reason the SWG table has 60-80 ppm CYA is that this helps reduce the breakdown of chlorine from sunlight letting you turn down your SWG somewhat and that also helps reduce the rate of pH rise. However, right now you are apparently fighting nascent algae growth so should take care of that first. I would shock with chlorine at a shock FC level according to the table based on your CYA level. That may fix your problem without needing the phosphate remover, but you can see and do what you think is best.

Richard