i use the amazonia NEW substrate in my 20L and my TDS and GH behave the same way. i keep it at 6dGH, my pH is slightly lower or around 6.5pH. I just top off with distilled water.
initially my tank was set up with remineralized DI water, then i switched out 50% of it with liquid rock tap water for some reason. that brought my TDS up to 240ppm. Now I'm just not doing anything with it really. if the dGH drops, I add more remineralizer to bump it back to 6dGH. my TDS fluctuates as plants and shrimp use it up and as i add more remineralizer. its at 257ppm now lol...
over a long period of time, say a month or two i think the substrate drives the TDS down on its own, so you shouldn't worry.