The CO2 chart is not accurate always. There are LOTS of things that can throw off the results (phosphate, nitrite, temperature, etc). When adding CO2 to the water, it forms carbonic acid. This lowers your pH, not your kH. I'm not sure why your kH dropped, but the addition of the plants might be suspect. It could also be an inconsistancy with your test kit (they are notoriously innaccurate also).
Here's the good news - you don't need to worry about getting a 100% accurate measurement! Tests will give you a rough idea (none, some, lots) of levels, but don't take them as gospel. Watch your fish, watch your plants - adjust accordingly. Do you water changes, dose your fertilizer, and you'll have a happy healthy tank without worrying about testing.
I highly highly doubt you have that much CO2 in your water (it would have killed off your fish 20 ppm before now!). The best way I've found to get the CO2 right when using a compressed system is to turn it up slowly over the course of a couple weeks until you see the fish gasping in the morning. Then turn it back down until they quit. You want as much as you can get without being harmful. With DIY CO2, especially in a 55 gallon, it will be very very difficult to come anywhere close to an unsafe level for your fish. In fact, I would recommend adding another DIY bottle. I use one bottle on my 15 and it is barely enough to keep the plants happy. After a few months of using DIY CO2 on a tank this big, I bet you'll want to start researching the compressed CO2. It isn't as tough or intimidating as most newbies think.
As far as your water chemistry, DO NOT add baking soda to raise your kH!!!! Use what you have - straight up tapwater. I bet every tapwater source in this country is suitable for growing plants, you just have to figure out which ones will work for you and keep your CO2 high and dose those ferts. Don't fight it - it will give you more headache that it will be worth. This hobby should be fun, wrestling with your water chemistry and trying to find some mythical ideal parameters using innaccurate test kits is NOT fun. Your tapwater is just fine (I know, I also live in Indy - we have hard water, but it grows plants great!).
That being said, you also have several things in your tank that will raise your kH. kH is also called carbonic hardness, because it is a measure of the carbonate in your water. Calcium carbonate is one of the main components in limestone and also in coral (not sure abotu tufa rock). If I were you, I'd take all the CaCO3 out as soon as you possibly can. The only exception to this would be if you were raising shell dwelling cichlids and wanted shells for them to hide in. Other than that, do not put anything that will mess with your water chemistry in your tank. You won't be able to control how fast it dissolves and it will cause you trouble down the line almost guaranteed.
I noticed that your pictures aren't showing up... try putting the url in between img tags, like this - [img#] YOUR IMAGE URL HERE [/img#], but leave out the # in the tag. I put it there just so the tags would show up.
If you live close to Indianapolis, consider swinging by the Circle City Aquarium Club sometime! It is a great group and there's no cost or obligation for showing up just to check it out. (Circle City Aquarium Club