pH and KH - Too much CO2? and other questions

I got a 55gal tank about a month ago. It has a AGA tripe-strip
fluorescent Iight (120 watts) and fluorite. The light is on for about
12 hours. I'm using an eheim ecco 2234 filter.

First I put in a couple of small amazon swords, a couple of anubias,
java ferns and anacharis/elodea. My tap water has a very low KH (1 or
2) and my pH registered at around 7.2/7.4.

After the first week, I bought some driftwood at a LFS. I asked the guy
if I needed to treat the wood before I put it in the aquarium. I know
some places sell driftwood already treated so it would not leech any
chemicals in the water. The guy said that I just need to rinse it and
that's it.
Well, when I put the driftood in my tank, the pH suddenly dropped to to
around 6 or lower (my ph test kit doesn't show lower than that) and the
water became yellowish.

Since I wanted to have tetras, I didn't mind the tannin in the water.
But I wanted to get my pH more stable so I added some sodium
bicarbonate. This brought my KH to about 4-5 and my pH to about 7.2. I
was happy again.

The anacharis started growing fast. The interesting thing is that the
new stems that grew from the old ones had their leaves more open,
sparser and lighter green that the original ones I bought. The anubias
were growing very slowly. The amazon swords shooted stocks and new
leaves. But the java ferns were not looking good. They didn't seem to
grow at all and started getting a lot of ugly hairy algae. I left the
fern that started having little leaves growing and got rid of the other
one that was full of algae.
I planted the java farnes on the driftwood. Were they too high and too
close to the light? Parts of the new baby leaves look transparent
green. Should I plant the java fern in the corner with less light?

Later on I got some dwarf hairgrass that I planted all around the tank,
a dwarf chain sword, corkscrew vallisneria and java moss. Not sure if
my moss was low quality. It was all brown and it doesn't seem to be
growing in my tank. I'm thinking about getting rid of the moss.

I also got 6 ottos. They're really cute and active little cleaners. Too
bad they don't eat the hairy algae. One of them died a day later. But
the other five are still there, happy and getting fat. I don't feed
them much, just some cucumber and algae wafer once a week.

And just recently, I started injecting CO2 with the hagen natural plant
grow system. (the yeast based with the ladder-type diffuser). I wanted
to start slow. They recommend one per 20gal. So since I have 55 I could
do no harm, right?
Well, right now, my KH is at 4 and my pH seems to be at 6.6. That's
about 30ppm of CO2.
My ottos seem happy and healthy. I can't believe that the CO2 system
for 20gal tanks is making such a big difference on my tank. My water is
not splashing by I have a spray bar right under the surface that moves
the water around.

Do I really have 30ppm of CO2 and should I be worried about my ottos or
could it be that my pH is lower for a different reason and my CO2 not
that high?

I also have a bunch of little snails. They don't seem to be eating any
plants yet. I get rid of them during my weekly water change (25%). But
there are more appearing. Are they dangerous to my tank? I don't mind
having them if they don't do any damage.

I'm planning to get some shrimp, rasboras, gouramis and tetras. I would
like to get neons or cardinals but if my water conditions are not too
good then maybe blood fin tetras.
Do I need my tank densly planted before I get the tetras?
Any recommendations?

Thank you!