A lot of LFS keep their tanks down around 1.017 - 1.018 without seeming to have any problems with fish; inverts I would say seem to prefer it a little higher. However, its important to note that recent studies have shown that apparently the ich tomites aren't killed until around 1.012 or so, which I would say is almost certainly too low. Values around 1.017 seem to at least slow the proliferation, but I wouldn't count on this alone to kill them off. Don't know if that's of any help, but there it is.