Replacement Filter for Stealth-RO. Stealth-RO150/300 RO Membrane reduces 89%+ of PPM of total dissolved solids. There are two indications as to when to change your membranes. 1. Test the RO and source/inlet water to see what percentage of the inlet water’s PPM the 150 ™ is filtering out (rejection %). You should see approximately 98% of the inlet PPM’s being removed when the membrane is new. If the rejection % falls under an acceptable level (typically 90%) it’s time to change the membrane. 2. When the flow rate of the product water slows down significantly. This can also happen if your pre-filters are clogged. If you change your pre-filters and the product water still flows slowly then it’s an indication you need a new membrane. When in doubt contact Hydro-Logic.