Ph going down after 5 years

I have noticed my PH going down from the normal 8.2 at 10AM to
8.4 around 11PM for the past 4 years. But over the past year
my readings have slowly gone down to 8.0 at 10am to 8.2 at 10PM
over the last 6 months its more like 7.8 at 10am to 8.0 at 10PM. I
recently did a 200 Gallon water change and the readings went pack up
to normal, 8.2 -8.4 but within a week or so it is coming back down
again to the 8.0 - 8.2 range.

I am using Tropic Marian salt which I thought had the best buffer
capacity but wondering if that is my problem.

Does the 500 pounds of sand in the tank have anything to do with the
buffer capability of the water? I did not think so, but wondering if
that gets depleted.

Of course Alk, KH go down as the PH goes down, if I boost ALK the ph
goes up it is just a temporary fix.

Any thoughts or comments would be appreciated.

Thanks
James