That's not too much of a TDS change, hence the fish should be quite unfazed about your experiments.

I'd monitor over the next few weeks, the KH, as well as the TDS value, and then settle on a base measure for both. As I said very early in the tread, use the TDS value as a measure of polution and keep the KH readable to prevent fluctuations. The soda bicarbonate should be adding a bit to the TDS, hence a bit of a difference with tap water. You just need to decide on a base measure, and not get it creep up too much over a long period of time...as that's when one slips up...You don't want any biological based pollution....It will eat at your KH value, and once its used up, the pH will crash down.
How are the fish by the way?