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pH changes with 'Nitragon' filter

I did something to my tank recently & I don't understand what happened.
Maybe someone can help me find an answer.

I live in Oxfordshire in England & have a heavily planted 55g tank (no CO2 &
slightly inadequate lighting - 1 x 40W PowerGlo, 1 x 38W 'Arcadia,
Freshwater Lamp') & a Fluval 404 filter (filled with GAC & bio-media).  I
have to cut-back my Vallis & Elodea about every 2 - 3 weeks to stop them
covering the waters surface.  I also have a selection of Crypt's, Anubias,
Java fern, micro-Sagittaria, Cabomba & Red Ludwigia.

It's also slightly (...) overstocked: 6 x Angels, 3 x Bristlenoses, 4 x SAE,
6 x 'Flying Foxes' (sold to me naively as SAE's before I knew better & now I
can't catch them), 7 x Otocinclus & 5 x Guppies. - All the fish are less
than 6 months old so not yet fully grown.  I also have an adequate
population of regular pond snails (accidental) & MTS (on purpose).

My local mains water is 50ppm Nitrate, pH=8.2 & very hard.  I dose with PMDD
(without KNO3) which works well for the plants but I have a pretty bad Algae
problem presumably caused by the high tank Nitrates (was >>80ppm) &
(assumed) high Phosphates.  The tank water has remained at pH=8.2 since I
got into this about 6 months ago.

Obviously I can't solve the Nitrate & (assumed) Phosphate problem by doing
water changes straight from the tap so I bought a 'Nitragon' tap water
filter.  It goes inline with the tap water when refilling the tank & it
does, successfully, remove 100% of the Nitrates (& I assume the Phosphates
as it says so on the wrapper).  There is no indication on the wrapper about
how it does this but it can be recharged by back-flushing with salt-water.

The interesting thing is that it also reduces the pH of the tap water to <7.
3 (the low-end of my high-end test kit) which resulted in a tank pH, after a
50% water change, of 7.6 (the tank Nitrates also dropped to around 40 -
60ppm).  This is not an advertised feature!

The thing I don't understand is that when I re-tested the tank water, prior
to doing another 50% water change about a week later (I'm trying to quickly
get the Nitrates down to around 20ppm initially & then I plan to try a
Nitrate removing resin in one of the filter compartments), the tank pH had
risen back to 8.2.

How could this happen?