From my experience so far using various brands of soil substrates that have pH buffering features... i've haven't experienced the pH dropping until 5.0 just by the action of beneficial bacterial breaking down waste material, even in an overstocked tank. Usually the large drops in pH are derived from too much Co2 injection.
There was a point in time i had like 80+ assorted fishes in a 2ft tank (had live-bearers which reproduced too quickly), it ran for a few months with aquasoil and the pH still maintained at around 6.4-6.6, didn't drop noticeably further throughout that time.
Its better not to add additional items to re-buffer the water as that may increase the chances of one buffer exhausting and the other buffer creating sudden shifts in water parameters without warning (especially with some brand of soil substrates which may have relatively short buffering lifespans).
I've found that for most fishes, even those which are higher pH/hard water species, as long as the changes in pH and TDS are slow and gradual, they can adapt to it over time and even breed in different conditions.![]()
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