ITS NOT THAT IT IS only FRESH... BUT THE STYLE OF FILTER IS NOT SET FOR SUPER WATER QUALITY, WHICH YOUR SALT WILL NEED. IT HAS A DISPOSABLE BAG FILTER IN IT, OR A THICKER BAG WHICH YOU FILL WITH CARBON OR SOME SUBSTANCE. THE PROBLEM LIES IN THAT WHEN YOU REPLACE YOUR FILTER AND PUT THE NEW ONE IN YOU BASICALLY WILL BE CYCLING YOUR TANK AGAIN. ALL OF YOUR GOOD BACTERIA THAT HAS GROWN IN YOUR BAG YOUR ARE NOW THROWING AWAY, OR IF YOU PULL THE THICKER BAG OUT (NOT SURE WHICH WHISPER YOU HAVE) YOU WILL BE DISTURBING YOUR BACTERIA BED WHEN YOU CLEAN OUT THE BAG ITSELF. THIS IS WHY I WOULD RECOMMEND THE BIOWHEEL FOR SMALLER TANKS WITHOUT A SUMP OR A HANG ON THE BACK FILTER SYSTEM. THE BIOWHEEL FILTER HAS THE BIOWHEEL ON THE FRONT THAT IS NEVER DISTURBED WHICH IS WHERE YOUR GOOD BACTERIA WILL GROW AND THEN YOU CAN CLEAN THE FILTERS ON THE BACK SIDE OF IT. THESE ARE THE SAME TYPE OF FILTERS THAT ARE IN YOUR WHISPER. SO BASICALLY WHAT YOU ARE LACKING IS A GOOD FIRM FILTER BED FOR YOUR BACTERIA THAT WILL KEEP YOUR WATER GOOD. THIS IS WHY I RECOMMEND WHISPER FOR FRESH WATER BECAUSE IT IS CONSTANTLY BEING CLEANED, MOVED, AND DISRUPTED. HOPE THIS HELPS....