My Red Sea hydrometer

[arandomguy.]

I got a hydrometer along with my tank and I forgot to get ro water to set it, is there anything else I can use to set it?

 

[Spare time]

Hydrometers do not need to be calibrated. If it is a refractometer (which I think is what you meant), the best practice is to use calibration fluid. You could probably use salt water that has already been confirmed to be at 1.026 or so, but if you don't have another means to check the salinity of the reference salt water, that would obviously not work.