in which smallest concentration is the plants able to fullfill their needs, nutrient wise? Lets say K. If I have 1ppm left before I add more, is it sufficient? Does any of the nutrients we add change formation/compounds within different pH intervalls?

Edit: I have pH 6.81-82 throughout the tank right now in the middle of the day with a KH of slightly below 7 (calculated and tested around 6.7). Do I have low CO2?
I work at a pharmaceutical company with access to pH-meters which are calibrated each day. The readings are correct.
The KH is tested with a simple bought test from the aquatic store, but increased 2 times for accuracy. If I calculate the local waterparams in tearms of alkalinity i get the same number. This reading can be faulty, but shouldnt be.

//Nicklas