I store mine in the garage, but I only make 60 gallons at a time. Thirty gallons are for my water change. Unfortunately, I'm still trying to use up the rest of my Red Sea salt. Once I mix it, I have to use it within 24-48 hours, or I get a brown film that gets nastier the longer I store it, but that has nothing to do with keeping it in a hot garage. It still happened when I kept it in the house. My top off water usually lasts two weeks, and I've not had any problems. I do have a powerhead running in both.