21,000 gallon pool with SWG (new this season) run for 8 hours a day.
I've had this problem with "pink algae" off and on ever since we got our pool over 6 years ago. I've used "Yellow Out" which was recommended by my local pool store with some success but the instructions tell you to basically shock the daylights out of the pool so that might actually be what is doing the trick!
This year we got a SWG and after a few initial problems (too much salt for starters) it has been working great. I got a DPD test kit and have been checking the FC and PH levels pretty religiously all summer. I've found that the SWG took a LONG TIME to get the FC level up to snuff but once it caught up I was steadily reducing the setting to keep the FC levels reasonable. Once I got the FC levels up using the SWG I stopped shocking the pool. The FC levels have been about 2-3 pretty much all the time since. PH has been consistently 7.6. The other day I decided to add some liquid chlorine to shock the pool for the first time in weeks. Added about a half gallon of 6% liquid chlorine. I'm always looking for signs of pink algae when checking on the pool and I found some just starting yesterday. It was showing up just around the edges of the return jets and I can see a little in the liner seams in the deep end.
I wonder if the problem starts less because of the chlorine level (which has been pretty high all summer) and more because of our constant use of the solar cover. It's been a bit cloudy and cool over the past week so we haven't been swimming much. The cover is a dark blue which probably doesn't let much sunlight in. I seem to recall that whenever we get pink algae it is usually close to the end of the season when we don't use the pool as much and the solar cover is on more to keep the temperature up.
I thought I'd post here to see if anybody else thinks the solar cover is a source of the problem. From what I've learned from this site, if the sunlight can't get to the water, then the chlorine can't be as effective at oxidizing the bacteria and the pink algae could be taking advantage of that.
I'll do a more thorough test tonight in case one of the other measurements is important (CYA, Alkalinity, etc) and post the results here. I suppose I should do a total chlorine test to get the CC number. That would be quite important here I'm sure.