It's hard to believe you can get that much co2 from 2 2liter bottles in a 75. My guess is that your ph or kh kits are off - I would look into that.
You say your nitrates have zeroed out. That's part of your problem. Get those up around 10ppm, get your phosphates around 2-3ppm and make sure you're dosing your micros. Your tank is out of whack (balance).
You've started towards correction - major water change, gravel vac etc. Then figure out what your real co2/kh/ph is, and start a regular dosing regimen. Good luck.