I'm looking into switching from freshwater to saltwater/reef. I will have some spare tanks sitting around and I plan on using some of them for quarantine tanks for frags and livestock, sumps, and to "make" my RO/DI water. I had planned on using 75 gallon tanks for both of the tanks I'm planning on switching (125 and 100). Can I fill up a 75 gallon tank with tap and then run the water through a good RO/DI system for a couple of days to get the water ready for the necessary waterchanges. Also, will the salt water mess up my septic tank?