It's not needed but it's definitely beneficial (depending on fish). Salt works best as a medicine when the quantity is raised, so keeping some in the aquarium at all times isn't a bad idea. The standard measurement is 1 tablespoon (approx 15mL) for every 5 gallons so I'd use just a bit less if used for an added benefit. Remember that NaCl doesn't evaporate either so when you do a WC just replace what was taken out. Also predissolve before adding to tank