Stubborn pH that won't go down

My pH is around 9 and I cannot get it to go down. In about a week's
time I've put in 3 bottles of Beckett pH Lower and now I've been adding
a granular pH reducer with no success.

The pond is roughly 3600 gallons. Even though the pond is about 3
weeks old and young ponds can show screwy pH values, I cannot explain
the failure to reduce the pH because:
1) Ammonia level is 0
2) Total Alkalinity is around 100 (the tester shows a color that's
about halfway between the one for 80 and the next for 120)
3) The rock border is not limestone. I don't know what it is, but I
ensured that the rocks that I bought were not limestone.

Sometimes the pH is a bit lower in the morning and I finally think that
the pH is finally going to a good value but in the afternoon I'm back
up to 9. It might actually be higher since 9 is the highest my tester
will go.

I have several plants that are potted in the sandy Florida soil and I
cover the pots with that standard tan gravel found at Lowes. Some of
my pots are on bricks, but bricks are made from clay and that's not
known for changing pH values like concrete block.

Some of my plants seem to be suffering from the high pH values. The
new leaves on my Water Hyacinths have turned yellow and some other
plants and aborb their nutrients directly from the water are not
looking too good.

I'm starting to get angry about this and I'm dumping larger quantities
of pH reducer in the pond. I'm afraid that something might be
preventing these chemicals from doing what they're supposed to be doing
then all of a sudden my pH will drop to 5!

What's going on?