Algae seems a constant in my pool (this is my third season with this pool, which is about 20 years old). I shock seemingly killing it all but it comes back within a few days. I keep FC around 3ppm and top up as soon as it drops below 2ppm but this doesn't seem to stop algae building up. Every time I brush the sides and bottom (weekly) a green film is visible so I shock again and it is fine for a few days. I've tried extreme shocking (>30ppm FC) for a week, flocing and vacuuming to waste but again within a few days of normal operating FC levels the green film is back. I replaced the sand in my filter this year but no change and I backwash regularly (every 2 weeks or if pressure goes up 5psi).
All other chemicals seems fine e.g. currently I have:
The water is clear right now and I'll add a little CC today and some liquid chlorine tonight but I'm sure tomorrow I'll see some green when I brush as it has been fine for 3 days now.
Any ideas why the algae is coming back so quickly and virulently? Should I use algaecide? Should I run with FC higher than recommended, say 5ppm? Should I switch to salt or add borates? The pool only gets light use mostly on the weekends.