When you can't discern the color, always assume the higher number, never the lower. Rough estimate on the java fern needed for a 55 to have 0 ppm nitrate would be approximately 12 plants that are 10-12" in diameter. I'd recommend getting the Seachem Nitrite/nitrate test. They are very easy to use, you can test both nitrite and nitrate with the same sample of water. The testing vessel is white, to make color discerning much easier. They are also more accurate I have found then the API test. I have 4 API master test kits, and used all four on nitrite and nitrate, plus the Seachem on all at the sametime. The API ranged on Nitrite 0-.5ppm, and on Nitrate it ranged from 5-25ppm nitrate. With the Seachem Nitrite was a definite 0 ppm, and the nitrate was ~8-9 ppm nitrate. All of the API tests are still valid according to their production date, and all were properly shaken.