I have been doing it for quite some time and high tide water seems crystal clear to the naked eye, but I highly doubt that there would be a high concentration of anything detrimental in 20 gallons of water and I now do it daily plus my tank is planted with macro algae and I test the water like its my religion, plus by adding ocean water your getting many trace elements(boron,fluoride,inorganic carbon,etc) you can not buy and also an addition of radiolarians and volvox and other microscopic organisms that make my tank thrive.