I've been working on this for 4 months. The nitrate filter pads don't seem to be good at removing it. Plants only use about 10ppm per day, but that won;t lower the nitrates over time. I think the best way to level off the nitrate is to reduce the ammonia to 0. This will limit the nitrites to the amount of ammonia produce by the fish every day. And, in turn, level off the daily nitrate production to the amount of nitrite produced by the ammonia the fish produce.
Weekly water changes of 40% pretty much keeps the NO3 level the same in my experience. This is saying that when I do a water change, the NO3 is 20ppm. A week later when I do a water change, the NO3 is at 40ppm before and 20 ppm after. You've got it easy with a 15g tank. If you changed 1/2 the water every two days, you should be able to get the NO3 down to the level you want it and only "spend" 45=75 gallons. For me with a 125g tank, 3 40% water changes costs 125-200 gallons.
There is a natural system call a deep bed where you have a dark and oxygen free gravel bed that's like 6-8 inches deep. This can cause the bacteria to use the oxygen in the NO3 and release N into the atmosphere for use air breathers. But with most aquariums, the bacteria are getting their oxygen from the H2O and not the NO3 because there just some much oxygen in the water.
So, water changes is the easiest and most effective way to remove the NO3 from the tank.