If you are going for ich cures, I would drip the fish slowly to 1.009. If you want to REALLY play it safe, match the salinity the fish is in, then, slowly, over 5 days or so drop it down to 1.009. When the fish is in 1.009 for a full 4 weeks, I would then bring it up to tank salinity over another 5 or so days.
If your just going to the main tank, which we all know is a big no no, at 1.018 I would do a two day acclimation from 1.018/1.021 and then 1.025.