I have yellow staining in parts of the pool. At first I thought it was a mustard algae and added a copper based algaecide. However, when I tried to brush down the sides of the pool the yellow would not brush off. I checked the pH and found it to be less than 6.8 (6.8 is the lowest I read) so I added sodium carbonate then shocked the pool. The next day I collected a sample and took it to the pool store for testing. The results were:
Free chlorine 5.8
total chlorine 6.4
combined chlorine 0.6
cyanuric acid 110
The stain appears pale yellow and is somewhat spotty around the pool. It is most prominent on the sides directly opposite the water return nozzles at each end. The plastic return nozzles themselves have developed a very heavy yellow to almost orange color.
Some additional information: The trees around the pool are all live oaks which keep their leaves all winter and start shedding them about the middle of march. Prior to shedding leaves the trees drop acorns starting about the middle of January and continuing until about the first of march. There are a lot of very large trees and they produced a bumper crop this year. So when the acorns start falling and continuing until after the oak pollen has fallen about the first of May it's not unusual to get a bushel basket of acorns, leaves, and or pollen out of the pool every day. If the oak debris is allowed to set in the pool for extended periods (several weeks) the pH will get very low and the water will be stained with tannic acid.
In addition to the large oak trees we live close to a wetland (we used to call it a swamp) that has a lot of large pine trees and they shed pollen about the same time as the oaks drop their acorns. The pine pollen is a yellow color and it covers everything. The surface of the pool will be a bright yellow every morning and the filter backwash will have a definite yellow color during the season.
Short of some very expensive testing I don't know of any way to determine what caused the stain. It could be pollen, iron, organics from the oaks, some combination of these, or something entirely different. My question is what if anything can I do to remove the stains without ruining the liner? I've brushed the stained areas thoroughly with no effect. I would hate to drain the pool completely and then clean it off, but maybe that is the only answer. I would appreciate any suggestions. Thanks.