MW9015 Storage Solution is the same chemical mix as the reference in Milwaukee pH probes meaning that reference will stay stable. Using storage solution is highly recommended and will help extend the life of your electrode.
Storing in or hydrating a probe with tap water can alter or weaken the probe reference reducing the life of the electrode. NEVER use DI (De-ionized), RO (Reverse Osmosis), or distilled water. Pure water will change the chemistry of the reference meaning the probe or pen will no longer function.