The reference is:
Krause, H.-J. (2007): Handbuch Aquarienwasser. Bede-Verlag GmbH Ruhmannsfelden, 6.Aufl., ISBN 978-3-927 997-00-5(book is in German language)
If you want to know more about conductance, search for “Square-root-equation Kohlrausch” and “Wheatstone bridge”
useful link in german: https://www.uni-ulm.de/fileadmin/website_uni_ulm/nawi.inst.251/Didactics/elekleit/html/index04.html