You can actually use the API liquid test kits, they work in both freshwater and saltwater. I was using them until they were giving me false readings because I have ammonia in my tap water and use Prime to nuetralize it. Prime (and similar products) from what I've read doesn't work well with the API kits because of the way Prime breaks the chloramines into chlorine and ammonia and then binds to the chlorine and ammonia so they're not toxic. The API kits raise the PH of the water in the test vial to 12 which breaks the ammonia/Prime bond giving you a false reading. I ended up buying the Seachem kits which also work in FW and SW...they're just a little more expensive and harder to read, but work with Prime.