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Chemically challenged question

A lurker's question (for the first time in a loong life I'm sorry I never
took general chemistry in college, organic is no help now :-))  I'm new at
all this and have something about my water I don't understand.  It is close
to being snow melt, comes from a reservoir untreated via a canal straight to
my house.  KH 3, GH4,  pH7.  I would expect a lower pH to start, then in my
small tank it goes up to 7.5 and stays there.

7 gal eclipse, eclipse light with some sunlight so fairly low light.
Potting soil under gravel substrate, weekly Flourish feeding with 30% water
change.  Also Flourish iron and occasional Excel.  Non CO2 injected.  5
Cardinals, 1 Oto, 1 SAE.  No visible algae.  Heavily planted with easy
plants, crypts, myriophyllum, Echinodorus, bacopa, ludwiga, java fern,
hygrophila, nymphaea, hydrocotyle.  All growing slowly-  however the new
leaves on the hydrocotyle are yellow with green veins and the bacopa, in too
much shadow, is olive green.  The val melted.  NH4, NO2, Fe all at
undetectable levels, PO4 at 0.2.  Poor little nutrient starved plants.

The pH worries me, I really want a lower pH so I can play at tinkering but
am afraid to add anything that would drive it higher.  Its already in a CO2
non-existant mode, I would think.  This is a learner tank, i plan to get a
much bigger one with injected CO2 but want some practical understanding of
the processes first.

Why is the pH neutral to start with in such soft water and why is it driven
higher in my tank?  I tried aging water out of the tank and it stayed at 7.
How does soft water get alkaline in this situation, and is there anything I
can do (other than injected CO2) that would help?  I have slowed the
filtration rate and surface agitation but am wondering about just not
filtering at all?  My search of the archives doesn't really answer this
issue; everyone else seems to have the opposite problem.

BTW, I do know dogs and the frontline and other topicals are absorbed in the
sebacious glands.  They affect only invertebrates, btw, like ivermectin.
Petting an animal and putting your hand in the tank should cause no problem
unless the animal is freshly dosed.  My very large dogs are in my smallish
farm pond all the time with no effect on the invertebrates.  Anyone need