I wonder what everyone's thoughts on using this reference solution as to "best practice". I cant find a specific instruction about avoiding contamination of reference bottle solution with older sample water already in the uptake tubing and white filter and also the other way round of leaving reference fluid in the filter as it is never fully cleared at the end of test!

Firstly is the reference fluid harmless to the tank?

presumably best way of avoiding cross contamination is to remove the filter and subtract 7m from tube volume! and dip the sample tube into a container with more than the "sample volume" say 30 ml? what do you do?

or do people just put the sample tube directly into the reference fluid?