I wanna do a water change this week and the closest local fish store sells already mixed salt water for about $4/ gallon. I know, its ridiculous. Anyways. The Petco right down the street sells a packaged already mixed salt water pack with 5 gallons of water in it. I think its made by Instant Ocean or one of those brand names. Any thoughts? Anyone see a problem w/ me using this? Should I not use this for a water change? I plan on getting a RO/DI unit, but waiting a little bit before I do. Thanks.