Custom Premix Selecting "Custom Premix" option means that all of the beads you have chosen will be premixed in manufacturing before the kit is sent to you.
Particle analyzers require water that is free of particles that will interfere with the accuracy of particle counting and sizing. Other parameters that indicate good water quality should also be taken into consideration, such as:
Bacteria-free Bacteria will be counted as particles, giving artificially high particle count.
Low organics, monitored as total oxidizable carbon (TOC) Organics serve as food for bacteria. Organic contamination could encourage the proliferation of bacteria, which could affect particle count data.
More Information
You may find more information related to particle size measurements in the following web sites: