what will co2 do to tanks chemistry?

CO2 will drop your pH. How much it drops your pH is determined by your KH level. You will need to raise your KH before you start adding CO2. I think the target level is around 3 or 4 KH

check out the carbon dioxide tech brief here

Let your tap water sit overnight before you test pH if you didn't. That seems like an awful high pH for the low kH. There are some types of water treatment that cause that sort of effect but if the water is aged, the pH drops considerably.

Some water companies add something or other to water to make it less acidic: less harm to the pipes. I'm sure the WetMan had something on it at the old Skeptical Aquarist but I can't find it at the moment. Its not my particular scenario, so I (unlike the WetFeller) haven't looked into it.

Its sort of an unusual circumstance, but its possible to have high pH and low KH. It just means that something besides carbonates are being used to buffer the H20. It could mean a phosphate buffer, for instance, which would have other serious consequences for a planted tank. Or it could be something else. The point is that its not normal: its something you're going to want to get sorted out. Its could be a faulty test kit, or it could be something your water company is doing -- I'd try calling them first. They may have a website that has a water quality report or you can ask for one when you call.

When CO2 is dissolved in water you wind up with H2CO3, or carbonic acid. Acid will react with whatever buffer it comes across, carbonate or not. If it burns through the buffer until there's nothing left you can wind up with a pH crash. You have a buffer now, you just don't know what it is or how to measure it. You need a buffer, but tossing in some carbonate source on top of your mystery additive may not be beneficial.

Aside from the whole alkalinity/buffering conundrum, your dealing with a chemical of some sort that may also have serious consequences for topics like nutrient levels and nutrient uptake.

Which is also where GH fits in: its a meaure of Calcium and Magnesium in your water. The plants need some GH for nutrients. It doesn't enter into the whole pH/KH/CO2 business. 4 degrees should be good.

EDIT: I was poking around a little more and it seems to me that acids are buffered by their own salts. For instance, carbonic acid reacts with a carbonate or a bicarbonate. They wouldn't necessarily go and react with a noncarbonate salt.

So what does this mean? I don't really know. Perhaps the WetMan or RTR or the PlantBrain or one of the mortals with a yen for chemistry will happen by and render guidance. I am at two with chemistry: not a clue…

The how:
Add enough baking soda to raise the KH to 3 degrees(~50ppm).
Add enough CO2 gas to get a pH of 6.5-6.6.

That's all there is to it. This is all you need to do.

The chemistry is adding more acid to an acid-base system. This moves the pH toward the acidic side.

Do not use other buffers/"acid" buffers to control pH.
You want to add enough CO2 for the plants, not some "acid" buffer etc that the plants don't use as a nutrient.

I measure out the baking soda in teaspoon measurements and simply remember hopw much I add to the make up water for my weekly water changes to get ~3 KH. It becomes old hat quickly.

You can learn the theory etc, but it's not required by any means to use CO2.

If you know how to add baking soda, measure pH and KH, you have it licked.
You need about 3KH or more for a good buffer to match the acid(CO2). Baking soda is your buffer(KH).

Your GH is fine.

Regards,
Tom Barr