Correcting green water caused by rain


Well-known member
Mar 30, 2017
My pool water looks green, don't know if its algae however, when i brush then can see brown dust in the water. Anyway decided to slam and the question i've is about Clorine/CYA chart here: Pool School - Chlorine / CYA Chart
My FC is 0 and Cya 50 so, based on the chart should i traget shock FC 20? What's shock FC? does it mean i need to bring my FC level to 20? If yes, then how would i test FC of 20? I ordered test kit from this site and i don't think it can test that high level of FC. So, can someone help?


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Rain does not cause algae.

Sounds like you need to SLAM. You used the TF100 to get your CYA, correct?

If so, you use your FAS-DPD test (DPD Powder, R-0870 and reagent, R-0871). Put 10 ml of water in the vial. Add one heaping scoop of powder. While swirling add reagent drops one at a time. When the sample turns clear, take the number of drops and divide by 2. That is your FC. See Pool School - Extended Test Kit Directions

Your shock level FC is 20 with a CYA of 50. Add liquid chlorine, based on PoolMath, to get to FC 20. Run your pump and brush. In an hour or two, test again, add chlorine to get to 20, repeat. After a day or so you will be able to stretch out your additions.

Can you put you what state, country, etc 'Frisco' is in?

Take care.


Well-known member
Mar 26, 2014
San Jose, CA
You simply don't have enough FC in your pool based on your CYA. It's as simple as that. Did you notice that the original poster got algae and he had 0 FC in his pool with a CYA of 50. If he would have maintained the proper amount of FC, he would have been fine. FC of 6-8 and never letting it get below 4 would have kept his pool clear after the rain.