Chlorine levels after SLAM


Bronze Supporter
Aug 18, 2018
Rolesville, NC
Pool Size
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-9)
I've had a mustard algae outbreak. I did a regular SLAM about 6 weeks ago and everything went great. When chlorine level drifted down after 2 weeks started noticing algae again. So 2 weeks ago did another SLAM but this time raised the chlorine level to mustard algae levels for 24 hours. Washed all the pool toys and brushed aggressively. After SLAM (confirmed negative OCT test) the pool again looked great but this time, I kept my chlorine levels in the high normal recommended range.
Latest test:
FC 8
CC 0
ph 7.6
Alk 60
CH 250
CYA 60

So according to CYA/FC chart, the high normal is 9

My question, is there benefit (since I have had such a hard time with clearing the algae) to keeping the FC at high norrmal levels or just slowly let it drift down to 3-5 at this point. Last time I did this, seemed to see th algae again. I have read some posts which suggested running the FC in the high normal range for a few weeks after a SLAM for mustard algae. Any thoughts ?


Gold Supporter
Bronze Supporter
Jul 17, 2019
Prosper, TX (DFW)
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
There is no harm in going higher than your target. My target FC on the chart is 5, but I'm usually between 5-9 just to be safe.

I would not let it drift down to 3, as that should be treated as your bare minimum as in never let it get to 3.


Bronze Supporter
May 27, 2014
I'm not an expert on here, but my understanding from years of reading on here and my own experience that I gained this year with a suspected mustard algae outbreak that turned out to not be, there is really no harm in keeping FC a few ppm above the recommended upper target level for pools as long as you have cya in the pool in so far as being safe and not causing other chemistry issues.

There are only two considerations that I can think of that you should be aware of if you decide to raise FC above the recommended target FC level each day for non saltwater pools, or try to maintain a higher-than-the goal level for a saltwater generator pool. This is especially true for manually chlorine dose pools, because the FC level recommendations are higher for non saltwater pools than they are for SWGs:

Firstly, and most importantly, if you stray above the target or goal level as your maintenance FC level, you'll use much more chlorine to maintain that level. The higher the FC that you try to maintain, the more chlorine that it'll take to do so. That's because the higher the chlorine level in the pool, the more quickly UV will break down that chlorine. So the higher level that you try to maintain, the more chlorine that will be wasted each day for nothing but solar burn off. In my case this year, I was keeping FC 9-10 instead of the recommended 7-9 for my CYA level of 60. Normally, in the middle of Summer, my ppm loss will max out somewhere between 2 1/2 and 3 per day, but this year during the middle of Summer, I was averaging 4 - 4 1/2 ppm loss per day just because I was trying to maintain 2 ppm higher than the recommended targets each day. So while this caused no harm, it did require much more chlorine to maintain that higher level.

The guidelines which has been developed and used for years: the chlorine/CYA chart; the minimum 7.5% FC/CYA and the goals which are 2-4 ppm higher than that 7.5% FC/CYA are carefully thought-out levels, and so while it is perfectly okay to try and keep FC a few ppm above that level, keep in mind, it will take more chlorine to do so, and the higher you stray from those recommendations, the more chlorine it will take to try and maintain the levels that you're shooting for. The best way to illustrate this is to understand what happens during SLAM. Since you've conducted a SLAM recently, I'm sure that you noticed that there was considerable FC loss during the day even after you got to the point where very little FC loss occurred during the night. This occurred because the FC level was maintained higher during SLAM, and therefore required several chlorine doses each day just to maintain that SLAM level. This daytime loss would occur even if you had no organics in the pool if one were to try to maintain SLAM level all the time; and according to what JoyfulNoise conveyed to me, this higher loss at higher FC levels will occur even without consideration of the CYA level.

Secondly, be aware that a valid and accurate pH test result requires that the FC level to be under 10 using the Taylor drop test kit that's included in the TF-100; and an even lower FC level is required for an accurate result with some of the cheaper test kits like Clorox. So, for instance, if you're trying to maintain FC 10-12, at some point you'd have to let the FC drop below 10 to get a good pH test completed.
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