If your tank is 46 litres, and the water in the tank is 200 TDS, if you remove 6 litres of water that's 200 TDS, and you add back in water that is 200 TDS, then the tank will remain stable at 200 TDS.
However, if the TDS rises to, say, 225 TDS, due to evaporation, food, waste and decomposing organics, if you remove 6 litres of 225 TDS water and add back in water that's 200 TDS, it's going to be less than 225 TDS but higher than 200 TDS.
Once you get the hang of doing water changes, you could add in water of lower TDS to target a 200 TDS count. Say, the TDS rises to 225, you remove enough water, and what you add back in is 180 TDS. Once the tank has had time to settle (you may not get accurate TDS readings once you add the water back in - it can take a bit of time to mix), the TDS will be around 200.