Swimming pools blog

I have a 30 x 15FT in ground liner pool with a sand filter, with new
sand 3 months ago. The ph is around 7.4 to 7.6 with chlorine quite
high at around 6ppm. I seem to have a constant problem with algae. As
I live in England, the sun is rarely intense, in fact rarely visable!
The pool is quite old, I guess about 15 years (we have had it for 7
years since we moved in). The guy in the pool shop suggested that the
problem was due to “old water” and that I should replace a
substantial part of the contents. I have done this but the algae
keeps coming back! I use granular floculant and vacuum to waste but,
within a few days, a green skin starts to form. Can anyone offer
advice how to cure this problem or any other aspect of the water
chemistry I could have wrong and should be checking?
Many thanks,
John Thomas