KH, GH, pH ... oh, my

Okay, so I didn't listening to JMK and I messed with my pond water
today. Sorry, ;-)

Reminder -- I have green algae water and test results yesterday we

pH: 9.5
NO2: less than 0.3
ammonia: 0
GH: 1.5 dH or 19 ppm
KH: 2 dH or 36 ppm
CO2: 0,1 (based on a chart analyzing pH and KH)

I added one cup of baking soda, then realized that if I did nothing to
bring the pH down that the baking soda would increase the pH. I have
muriatic acid on hand (for pool and cleaning aquariums) so added ~6
ounces. After two hours did the following tests:

pH: 8.5
GH: 3 dH or 54 ppm
KH: 5 dH or 90 ppm

So if I add another cup of baking soda tomorrow, the GH and KH will /
should increase again, but because I'm stabilizing the pH with the
baking soda it shouldn't increase and might even decrease? [Before I add
anything, I'll retest pH, GH, KH and ammonia in the morning.] As I
lowered the pH today (and the GH and KH increased), the water seemed
clearer. Does high pH water hold algae or other fine particles better
than lower pH water?

If GH is a measure of calcium and magnesium, why does it increase by
using baking soda? [Little rusty on chemistry, especially at 12:02 am.]
--
Kathy B, zookeeper
3500gal pond, 13 pond piggies
Oregon, Zone 6