Nutrient levels

Hi i have a simple question i need answered :confused:

This is my first time out on hydro (i have only done soil before), i brought a truncheon to measure my nutrient levels and i was wondering why it was showing a reading in plain water, do i have to minus this reading after i have added my nutrients from the reading then. I take it that the truncheon is reading the impurities in my tap water. (i know its a simple question but i cannot find an answer), thanks ....

after you add the nutes,adjust the the ph to 5.5-5.8,then check the tds again,then add or extract or dilute to the target measurement....bonbon

Thanks , that clears things up, i will post some pics as soon as they are on there way, nice one. ryder
:)