hi Alex,

very bright halogen light), and when in doubt, I always create the
control
specimen, which is measuring the pH of my tap which is really high in
pH.
there is a marked color darkness difference between very high pH (using
tap
water), which does reflect the 7.6 color on the chart. but what i get
is

The most likely reason that your tap water has a high pH is that it's low
in
CO2. The CO2 level of the water straight from your tap might be variable
at
different times. That's why such a reference sample is unreliable. You
should let a cup of water sit for a while, or aerate it, before taking its
pH. At KH of 2 degrees, your pH at equilibrium should be about 7.2. That
applies to both tank water and tap water.

the tap when left out for over 24 hours, the pH reads about 7.8... we think
the municipal water department adds something to make the pH artificially
high... the tank pH is lower than the tap (even after left out for several
days). probably because of the fish and the bog wood and stuff...

good to know KH=2 stabilizes at around 7.2...

How wide can the plant-respiration-induced pH fluctuation be? can it
really
go from 7.0 to 7.6? (37G, moderately planted...)

It sounds like you have plenty of agitation. But if there wasn't enough,
the
plants could easily bring the pH up to 7.6. That only equals a 2 ppm drop
in
CO2 from equilibrium. Of course, the plants would consume more CO2 with
higher lighting. How much light do you have? Any sunlight entering the
tank?

it has one 55W CF through a glass hood and is on about 12 hrs (11am to
11pm.. so there is about 5 hours of "not pitch dark" ambient, filtered,
indirect light into the room prior to the lamp going on). so the tank is
about 1WPG (assuming about 30% loss in light intensity through the glass).
the tank is in a well-lit room with lots of windows, but the tank itself is
about 8ft from the closest window. it used to get a short direct light just
before sundown, but right now I have the drapes so it doesn't get any direct
sunlight (my feeble attempt to control algae..) for the past few weeks. the
room is still quite bright though (lots of windows).

As I said, it seems like there is more than enough agitation from those 3
sources.

:-) fun watching danios surfing the current.. whhhheeee!!!

no. GH went *up* from 2 to 6... and no.. I have no rocks or any

Okay, I misread that part. It might be due to snails dying and their
shells
melting, but I'm not as certain about that. Or it might be the way you're
reading the GH kit results. I have a hard time with my AP GH test kit. The
color change from greenish-yellow to green is too subtle. Sometimes I miss
it and keep adding drops of the test solution.

yeah. I used to have that problem until now I look at the color by looking
directly into the tube from above.. This seems to give me better color
distinction just at the titration point. usually after two drops, the
yellow/orange is so faint, I can't tell when it turns green.. but these
days, more drops I put in, more yellow/orange it gets (and I shake the tube
using the cap too...).. so there is no mistake that GH is much higher than
it was before... it's just.. strange...

Anyway, I think a large water change would greatly help reset the water
parameters, and balance things out. Change perhaps 50% of the water, and
see
if the GH, KH, or pH start to drift again. Make sure you take the pH

ok.. this weekend is my maintenance weekend, so i'll do larger water change
than usual (usual is about 25%)

readings at the same time each day. Then test the water at different times
on a single day to see if there are any daily cycles.

good point. I usually test in the evenings (after I get back from work).
I'll try a few days in a row, at the same time, and over the weekend, I'll
try morning, midday, evening test to see if I see anything..

wouldn't adding baking soda increase the pH of the water? if the water
is
already at or close to 7.6, I don't think it's such a good idea to
increase
pH further (esp for the fish)... are there other means to increase KH
without increasing pH?

Yes, more KH will increase the pH, but if the water is at 7.6, it's for
some
other odd reason. If the KH is 3, for example, the equilibrium pH at that
level should only be about 7.4. But find out exactly why it's 7.6 before
adding the baking soda. A way to increase the KH without affecting the pH
is
of course to inject CO2. The addition of CO2 can offset the pH increase
due
to higher KH.

We currently think that the water dept adds something to make the pH
artificially high for the low KH value (prevents corrosion? may be it keeps
copper from leeching toxins or something? I don't know). I don't know what
they add to increase the pH. when I test, ammonia, nitrite and nitrate is
zero in the tap... I don't have any phosphate test kit or any other
indicator to figure out waht they do.. may be I should ask.

linda