My tap water has a KH of 2, so I use Baking Soda to increase it to a KH of 5. Doing approx. 50% water changes weekly, I find that I must use 1/2 to 1 tsp of Baking Soda to keep it at 5. That tells me that the tank water is not lowering much, if at all.
I also use Baking Soda in my pool (though, obviously substantially more) to keep the alkalinity up. I've never found that it lowered once I get it stable at the beginning of the season. But, of course, a pool is an entirely different situation.