What is this "light green" blur/stain?


Active member
Dec 30, 2015
San Jose, CA
From time to time, at 1 or 2 corner slots of my pool, there was a small area of "light green" blur or stain right on the surface of the vinyl, I don't know what it is, or anything algae related. Every time I used a pool-brush to wipe it off.

Everything else is fine, the water is clean and blue, latest test result is below:
FC: 5
PH: 7.5
TA: 70
Calcium Hardness: 80
CYA: 50




Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
Houston, Texas
It is probably algae. 5ppm of FC is at the low end of your FC range. Trying keeping the FC around 8ppm which is the top of your recommended range, and try to adjust the returns so you get better circulation. Usually when we see areas in a pool that continually see algae its a combination of low chlorine and poor water circulation. Brushing the pool after adding chlorine will make sure the chlorine gets to the area where the algae returns.