What's the pH of your tap? If it's higher, then it's likely not CO2 causing the issue - which is a good thing...most municipal water shouldn't have a CO2 problem.
I used to burn through mixed only slightly under that rate - 60 gallons and I had two smaller canisters of mixed exhausted. High pH (8.2+), High TDS (400 ppm), Chloramine city water.
I installed the 3 stage DI from BRS and now only replace Anion media about every 200 gallons. I have yet to replace the Cation or Mixed after 400 gallons. It might not be great, but I'm much, much happier and the cost is much less.