If you are a beginner in this hobby and have a small fowlr tank, you will want to go with a hydrometer for price reasons. You should not do this. Hydrometers are not extremely accurate and can cause you to add more or take away salt. Go with a refractometer. They are accurate and will give you correct readings. One time, my hydrometer stuck and would not work. That is when I decided to go with a refractometer.