I've been stumped the 2nd half of this pool season with dead algae settling on the bottom of my pool within 3-4 days after vacuuming. I've been a dedicated TFPer for 5 seasons now so I know that, more often than not, this is actually a chemistry issue rather than one of filtration, but I have done so many things and am pretty confident that I've ruled out chemistry and concluding that it is filtration, so I need some further advice.
Here is how I've reached my conclusion:
My water remains TFP clear - guests come over and at first glance think the pool is only half full. I know TFP clear water, and I have it. But junk (dead algae I believe) settles on the floor throughout the week, I vac Saturday morning and it's clear all weekend, until Wed/Thursday when it starts to show up. This has been ongoing since early July. I am not in a dusty area, so do not believe this is external.
To rule out chemistry issues, I bought some of the R-7065 about a month ago to confirm my CYA. This stuff looks like more like 35-40 to me and not 50, and my pool water looks like 35-40 to me as well, so I've been assuming 50 for my CYA level. Went through the SLAM process (including brushing) passed the OCLT the first night, but continued for another day, passes the OCLT again. I test regularly and have not seen chlorine usage outside of expectations - losing only 1.5 or a bit less on days (usually going from 8ppm to 6.5ppm) with no swimmers, and maybe 1.5 - 2 on days with swimmers. I have run OCLT somewhat regularly and never fail, often showing no loss overnight. My regents are new this season.
Since I began treating my CYA level as 50, I have never let my chlorine level drop below 6, never. CCs show up only occasionally, and it is always 0.5 or lower.
I've checked the usual suspects by removing and brushing the stairs, and leaving them out for an extended period of time. The only light I have is on the return. I've also left the pump running non-stop for weeks at a time to see if that changed anything. It did not.
I use a cartridge filter, media bought new early last season. I've inspected it for tears, holes, etc and see nothing. I am very careful about ensuring that it is seated properly as well. When I take it out to wash, I do notice that it feels slimy in places, could algae be somehow surviving inside the filter at an extremely low level?
The only other hypothesis I've come up with is maybe the filter housing is so old that the seating is out of round and thus dead algae sneaks through over time and gets sent back into the pool?
What else can I do to try and diagnose the source of this? My next step would be to purchase a new filtration system, or at least replace that old fiberglass housing that scratches me up every time I break it down.
Any thoughts? Something else that I did not think of? Please help!