I've read that the sediment and carbon filters do nothing to reduce your TDS, so you won't notice an increase in TDS when those filters are ready to be changed. I think it's best to stick to a schedule and replace them regularly. I change them every 6 months. Fortunately, they're not expensive...
The stand looks great! Coincidentally, I am about to embark on a refinishing project myself. Like you, I got a great deal on a used 90 Gallon Oceanic with oak stand and canopy. But unlike you, mine came already finished in black, and my wife wants it stained to match the rest of our furniture...