Interspersed comments...

|| Newbie to CO2 injection needs help.
||
|| My setup:
||
|| 10 gal. freshwater aquarium
|| 2x 23W 5000K lights (4.6W/gallon)

a bit high on wattage, 1 23W would be ok, unless you're growing Ricca or
glosso...hahaha....

|| Natural gravel substrate
|| Assorted plants and fish
|| Ghost shrimp
|| Malaysian ramshorn snails
|| Aquaclear filter
|| DIY CO2 injection (6 bubbles/minute into filter)
|| Airstone (used when lights are off)
|| Heater (75°F)
|| PMDD
||
|| My tapwater is around 8.4 pH (as tested with a high-range kit). The
|| CO2 lowers the pH some, and I lower it the rest of the way to 7.0-7.2
|| using dilute hydrochloric acid. A bit unorthodox, but I will never
|| use pH-Down or any other phosphate-based buffer again -- I like to be
|| able to see the contents of my aquarium, not just algae. I don't
|| currently know the GH/KH of my water, but I assume it must be pretty
|| high.

You really should find out the kH and gH of your tank... they play a vital
role in your CO2 production and pH problems... use Chuck Gadds excellent
calculator or website to accurately find your CO2 levels, using your pH and
kH http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm which is on this
page http://www.csd.net/~cgadd/aqua/articles.htm

The more CO2 you get in your water, will lower your pH... using the acid, is
out of my league... no pH down either....

|| Plants are growing a bit better, with a little pearling at the very
|| top of the aquarium close to the lights. However, CO2 still seems to
|| be a limiting factor. I know this because I used a bad CO2 recipe
|| the first time, which generated far too much CO2, crashing the pH
|| below the bottom limit of my test kit. The result was insanely fast
|| plant growth, and pearling all over. Unfortunately, the snails were
|| suffering rapid shell damage. (The fish seemed unaffected.)
||
|| I want to see that awesome plant growth again, but I don't want a
|| bunch of naked snails, so:
||
|| 1) At how low a pH can the malaysian ramhorn snails thrive on a
|| permanent basis?

permanent? I'd say 6.8 but you will see calcification of the shells... you
can add calcium bicarb to help I believe..

|| 2) Will it benefit the plants if I add more CO2, and use less
|| hydrochloric acid, while maintaining the same pH? Or is the CO2
|| already sufficient, but not directly available to the plants because
|| of my current pH of 7.0-7.2?

If you don't know your kH, you don't know your CO2 levels... you must know
your CO2 levels to adjust your pH.. see how it's all sorta connected???
What you need to do is get a kH test kit... Use Chucks CO2 chart to see
where you are... then figure out where you wanna be....

|| And a few other questions:
||
|| 3) How do you acclimate fish to a CO2-injected tank? Normally, I'd
|| do a slow acclimation over a few hours by adding tank water to the
|| container the fish is in, but with CO2 leaving the water and raising
|| the pH once it leaves the tank, I'm not sure how well this would work
|| anymore.

I acclimate the same way, just longer... add tank water to bag, ever 15m or
so. after an hour, dump 'em...

|| 4) After the accident, I was *not* able to bring the pH back up after
|| removal of the CO2 and vigorous aeration over two days. It took
|| water changes to bring it back up. Did I cause some non-reversable
|| chemical reaction?

You probably killed your bio filter, bacteria doesnt' grow below 6.2-6.4 or
so, which is probably why you had a crash...

|| 5) Though I don't have this problem right now, could you cure an
|| aquarium of a pond snail infestation by simply lowering the pH with
|| CO2 for a few weeks, destroying their shells?

you 'could'... but at what cost?? You'd lose your bio filter, and if there
was anything else in the tank, it'd be belly up too

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is that better??