With all instruments you need to calibrate. The Milwaukee you zero which is essentially a calibration. With an optical refractometer you need the calibration to make sure it is accurate. The calibration solution is a known specificgravity, typically around 1.026. Before you use it you test it with calibration solution. If the specific gravity is correct you don’t need to do anything if it is off there is a screw you adjust until it reads correctly. It isn’t difficult and the instructions they come with are usually pretty easy To follow.
I wouldn’t get a giant moray anyway your tank is too small. I’ve had snow flake morays before. They are super easy And aren’t fish eaters. Stick to a smaller species of eel.