you can use tap water that hasnt been sitting out....i do all the time and have no problems (well currently, i did before because i followed the label on some nutes and burned them). Your really not doing all that much by leaving it sit out, yes, it will bring the pH down some, but its not gonna be enough to worry about. You need to keep flushing those plants until the water coming out of the bottom of the plant reads 6.5 - 6.9, after that is completed you plants will return the love in about 24 - 48 hours (you should see a difference that quick in soil) You dont HAVE to repot, it is recommended, but not a necessity, you will just have to remember to watch that soil pH closely.