More or less any one will do as long as it is is temperature correcting and meant for a saltwater aquarium. There are many others out there for various fluids, but they have the wrong scales. It's also useful to have one with a scale that goes to ZERO so that you can calibrate it with pure freshwater. I think most for aquariums are like that, but I've heard of a couple over time that used a narrower scale. Anyway, I have a no-name brand refractometer that has served me well for several years now and has been very sturdy. They are pretty simple devices really.