algae bloom after heavy rain event

Well thanks to this forum I have recovered very quickly from an algae bloom this week.

The Fc of my pool was a bit below the recommended 4-8 (maybe 3.5) at the time of the rain event (about 35mm) and I was away from home for a few days. When I returned home on Monday afternoon I took off the cover to find that the pool was green. I had just got my Taylor K-2006 kit last week form a pool store in the US ($35 cheaper than the "local" supplier even with postage included ($35)) The "local" supplier is about 1500 miles north of Melbourne so not really that local.
Back to the plot.
Turned off the SWG, checked the PH (7.5) brought the FC up to 35 with Dichlor, left the pump running 24/7 and rechecked and adjusted the FC back to 35 every couple of daylight hours. Its now wednedsay evening and the pool is almost crystal clear again. I have brought the FC up to 35 again and will retest in the morning if it is still between 34 and 35 I think I am done and will let the FC drop back down to about 6.

Last test results
FC 19 (added Dichlor to get it back to 35)
CC 0.5
PH 7.5
TA 100
CA 140 (a bit too low should be 260 from PoolMath) I will fix this on the weekend.
CYA 80.

I am a new pool owner so still learning, but this site has helped me a huge amount in sorting out this little episode.

Thanks to all who have posted their wisdom here.


TFP Guide
In The Industry
May 14, 2007
New Brunswick Canada
Yep, had the same thing happen to mine this summer, warm rain, temp was 29c and my system only runs 8 hours a day. My swg puts out 3ppm in 12 hours in my pool, just wasn't enough and it went cloudy by next morning. 2 bottles bleach cleared it up in 10 hours. I got lucky or it would of went green lol. Swg's are great but as Chem Geek has taught me, you really need to know your systems daily output in ppm.


TFP Expert
LifeTime Supporter
Jun 7, 2011
Midland TX
Welcome Mike!

I always have a short answer for the rain event problem.

If you turn cloudy or green after a rain, and did not have significant run-off into the pool, you simply did not have things in line when it happened.

Sounds like you are heading in the right direction, but watch out with the DiChlor. You're at 80 already and high Cya gets people in trouble more often than not. Ok with the SWG, just a reminder to watch.
Yes you are completely right , things were not as they should have been. The FC was down at 3.5 before I left and the SGW was probably set too low for the time that the filter runs each day, so with the addition of the rain and some warm weather (pool temp is 28 deg C) it went green. I know what I need to do now so hopefully it wont happen again. I will lower the CYA back to about 50 after this weekend as we have a bit of a family get together on Saturday and the grandkids will be in the pool and probably some or all of the parents and grandparents too.


TFP Expert
LifeTime Supporter
Aug 17, 2012
Elverson Pa.
I wouldn't lower your CYA level. For a SWG we recommend it to be in the 70-80 range. What's your CYA level now ?
Either increase the pump runtime or increase the SWG output. 😎
it was closer to 90, but i refilled it a bit, because I emptied a bit too much out after the rain filled it up.
I will check it again tomorrow and see what it is like. its hot here today (32 degC) so the kids will be in the pool soon. Cooler tomorrow about 25degC.