I have a SWG (IC-20) and my CYA is around 35. The pool store and builder say this is fine. But Pentair says it should be much higher...in the 60-80 range.

Should I increase this? I notice over the last week or so my FC has been dropping and I've been having to increase the output on the SWG.

Welcome to the forum Yes, you should increase your CYA to around 60-80ppm. You can find suggested levels for all your chemistry in Pool School in the upper right hand corner.

Welcome to TFP!

The higher CYA level is needed if the pool is exposed to sunlight. If it is an indoor pool or is always shaded, then the CYA need not be as high. Dave assumed your pool was an outdoor pool exposed to sunlight, and most likely that is the case. I just wanted to let you know more specifics if this is not the case.

While the recommendations you've received are quite valid, a contrasting point-of-view is that when I maintained for the first 12 months CYA at around 70, with IC-40 running at about 20% and runtimes at around 8 to 10 hours per day, small amounts of black spot algae would develop unless brushed every other day and FC stayed at around 4 to 5.0 each day. When I allowed CYA to drop to around 20, the same runtime, same FC levels(also same pH, TA, CH), the black spot algae disappeared

The recommendations assume good circulation, non-pitted pool surfaces, and elimination of nascent algae prior to maintenance levels. If one has a pool that has existing black algae growing in deeper pits or yellow/mustard algae behind light niches or has areas of poor water circulation, then a higher FC/CYA ratio would be needed to prevent algae growth. That can be accomplished through a higher FC or a lower CYA, but doing the latter would usually require the SWG to be on longer in a pool exposed to sunlight and would likely have a faster pH rise and more acid needed as a result.

Of course, everyone's situation is unique so while most pools operate well at the recommended levels, one can adjust accordingly. Susa, your post above is the first one I see of you mentioning the black algae problem or your changing the CYA level down to 20 ppm. At this time of year, the sunlight is less intense and not all days are sunny so having a lower CYA level can be OK, but during the middle of the summer let us know if you are able to keep a reasonable on-time for the SWG and whether your acid usage changes.