If your only other alternative is to mix up tap water then I would opt for the premixed from the LFS. Is this fresh premixed saltwater or is it straight out of the tanks at the LFS? This may be a silly question but I have seen water sold straight out of livestock holding tanks before. I would not buy water straight out of a livestock tank because of numerous reasons but most of all because of risk of introducing disease or parasites. Save up for a RO/DI unit and a rubbermaid trashcan. It will be as good of an investment as buying a good skimmer.
