Question about top-off

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.

Diana said:

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.

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Bummer ,I felt that coming....my tap water is 15-GH and 10-KH.Measured with drop tests from Tetra,several times .Water company chart says GH is around 20....oh well ...
TDS is around 400.
Should I stick to just top off then ,and avoid the classic weekly WC ?
I don't use CO2 ,nor Excel ,and I'm lightly stocked.The truth is I feel unconfortable breaking this routine ,like many others ,despite being probably the best thing for my tank.
Until now ,I would change 40 % of my water weekly ,mainly to vacuum the bottom - more vacuuming ,more water syphoned.I am stunned by some low tech/low maintenance tanks I see on this forum ,and it's hard for me to believe that the owners change water every couple of months.