I had the same problem when I first started my 15gallon. I was going through almost 1/4 tsp of baking soda a day, just trying to keep the carbonate up. What I ended up doing was letting the hardness and pH drop, and doing frequent small water changes. Eventually, whatever was chewing through the carbonate ran out, and it's been fine for months. Recently, I got a bit behind on water changes, and it dropped back again, so I'm playing catch-up right now. Hope this helps some.