Hello,
I understand the discussion in that way:
tds=all minerals
conductivity=all substances conducting electric current (including humic acids, H+, nitrate)
–> conductivity is always higher than tds – is it right?
I use the conductivity to check when water must be changed. For example if the value of 40uS is reached (RO water pure: 15uS). In some auariums the conductivity declines very slow, in this case I do not change water. If tds is not effected by Nitrate, conductivity is the “better” method.
Greetings Patrick