I second this. I mix hw reefer at room temperature. I have 1 pump at the bottom of my 55 gallon mixing barrel and a small Powerade near the top. I heat it the day I'm going to use it and never have a problem with it.Don't store your SW unless you are NOT heating it and circulating it with O2 ... Are you using Store Bought RO Water ? If you leave DI'd water SIT it will collect air born elements. remember RO DI Water is a SPONGE to absorb anything..
Still using HW Marine \ reef here .. never issue ..