What to rely of: GH &KH test kit or TDS meter:
Mmm, it could be looking for troubles, relying on TDS alone, instead of GH, pH, pH and whatever else is in it.
You have to have a buffer to keep pH stable and water hardness within species requirements. After someone else says an opposite, do yourself a favor and check by yourself.
TDS meter related:
If you still rely on TDS, get a reliable TDS meter in above $100 range, as Milwaukee brand, and regularly calibrate it.
Or, to know why, spend the same money or 3 common handheld TDS meters of most popular brands, calibrate them and measure the same water. In my case, difference was in two digits, not something to rely on for fine tuning.
Using calibration solution in closest to a target range (150 ppm instead of more common 342 ppm) could make it more accurate, but it is cost prohibitive in my area.
Know what you are doing, part 1:
Search and read before relying on advice from LFS to avoid unexpected consequences.
Adding RO water dilutes tap water proportionally, lowering GH and KH. You can lower it too much, getting troubles.
This could be preventing by monitoring results of your doing by testing for GH and KH. API has a liquid test kit for it, 1 drop = 1 dH.
More reliable way is to create optimal water as a water for water changes, diluted or fortified as needed, then only the same water goes in the tank, water parameters are stable, what is more important than "chasing numbers".
Know what you are doing, part 2:
Using water water instead of tap water: first you are cleaning water by using RO filter, then undoing that by using a waste (brine) from it.
Skipping this all together leaves unmodified tap water, no muss, no fuss. If only chlorine or chloramine bothers you, water conditioner works reliably for this.
RO water is used when it is really necessary or really wanted, to dilute tap water or switching completely to a remineralized RO water when tap water is of unacceptable quality.
Remineralization is done with GH/KH+ for inert setup, as a water for water changes, each time. This is a work to do, eventually you will get used to it.