If your tap water is hard, high TDS it would be much better to use RO for top off.
My tap water is fairly soft (GH and KH 4-5 degrees, TDS about 280 or so) and when I JUST use that for top off the stuff builds up slowly over time.
My tap water is fairly soft (GH and KH 4-5 degrees, TDS about 280 or so) and when I JUST use that for top off the stuff builds up slowly over time.