The most suitable replacement for this is the
H2O International H2O-RC
We learned in February of 2008 that the NSA 25I has been permanently discontinued. Further, NSA has sold it's customer list and exited the water treatment industry altogethor. Once the industry's largest brand is now an orphan.
The company NSA sold to offers the same line of H2O International products that we have been recommending as replacement upgrades for NSA filters for 13 years, but at much higher prices than ours.