the tap is high in pH (about 8 when fresh out of tap, after overnight
settling, it is about 7.8). since the KH is low, the current thought is
that the water dept adds something in the tap to keep the pH high.

Can you ask them? Or search online. Many localities have their water reports
on the net. Search on the name of your locality and "water report"

And are you sure your KH test kit is working? Does adding baking soda increase
your KH?

Tank is can be regarded "moderately low light". I have ~5400K 55W CF (37G)
with the metallic reflector, sitting on top of a glass hood (Versatop).

So it's 55 over a 37 gallon tank, plus natural light?

That isn't a lot of light, but some people have had pH problems even with
moderate light. (Probably it depends on how many plants you have. Plus, the
first couple of weeks, PCF bulbs are extremely bright. They dim down a lot
after that.)

Try measuring the pH in the morning, just before the lights go on, in the
middle of the photoperiod, then in the evening, when the lights go off. If the
pH is lowest in the morning, then rises all day to reach a peak at the end of
the "day," then the plants are the cause of your pH changes.

If the pH gradually rises over several days while the KH drops, again, it's
probably the plants.

the gravel I use is supposed to be inert
(50% epoxy coated, the other 50% is natural)

The epoxy coated should be fine, but what's the natural gravel?


Leigh

http://www.fortunecity.com/lavender/halloween/881/