Strange TDS readings?

[Aqua sobriquet]

I use mostly rain water in my 60L and 37L Nanos. Before using it I filter it through some kitchen towel to get any of the larger particles out of it.
I just checked it after filtering with a TDS meter and got a reading of 23ppm. Out of curiosity I checked both Nanos, the 60L read 280ppm and the 37L 180ppm.
I then checked water straight out of the tap - 320ppm. I’m not surprised at the 180 reading on the 37L but I’m somewhat surprised at the 280 reading on the 60L as I don’t normally mix much tap water with it.
I am using carbon on the 60L to take out the tannins from the bog wood and it’s doing a good job. Could the wood still be the reason for the high TDS reading though?
I also, out of curiosity checked the water from our kitchen filter tap and that came out at 170. We don’t generally use it very much and the Brita under sink filter hasn’t been changed for years!
Both tanks have cherries and the 60L has half a dozen Corydoras.

 

[Hanuman]

Could be an unexpected fert overdose, substrate leaching out, or that the day you added water from the tap it was higher than the normal reading you get. This can also happens when you disturb the substrate a little.
I just re-did a 60cm tank I have. I have only been using RO and haven't been adding any sort of remineralizers or fert. TDS goes up ~50TDS points everyday. The first day it was even worse, it went from 150 to 600TDS in 24 hours. I am pretty sure that's the substrate leaching all kind of minerals.