Your numbers don't make sense. First off, be sure your test kits are accurate. Prepare some standard solutions and see how accurately they read. Keep in mind that there are two ways to measure hardness - in degrees and in ppm (parts per million). To get to degree from ppm divide by 17.4, so your gh of 200 converts into about 11.5 degrees of hardness for gh.
You're using Eco, which typically will RAISE kh, not lower it. So your kh dropping makes no sense! This, again, makes me wonder how accurate your measuring kit is.
If you have city water, just get a report from the municipality. It will tell you all you need to know, and then some, about your tap water.
As far as raising your kh for CO2 dosing, this is a belief that is quickly losing followers. There are many hobbyists out there with kh1 water who do not add bicarb to raise it, and have great tanks, and happy, healthy fish.