What did you end up using to measure salinity??
O.S.T. stands for Osmotic Shock Therapy which is commonly referred to in the hobby as hyposalinity [water that is deficient in sea salt]. Essentially, O.S.T. simply places the infectors [Cryptocaryon parasite---ick/ich] in an environment in which they cannot survive while the host, (or infected fish) can. This remedy WILL NOT work in reef systems, invert tanks or FOWLR as it incorporates lowering the specific gravity of the entire system to 1.009 Specific Gravity which is not tolerated by inverts or LR.
To drop the salinity, this is done as you would do a normal water change. However, you are simply replenishing your tank with fresh RO/DI water---not salt water! Monitor the lowering closely so as to not reduce it too fast. Usually over a period of 48 hours is fine. The bacteria colony which is the biological support for your tank will survive, the fish will be perfectly fine, but the ich will not. By lowering the salinity, you will also be lowering the osmotic pressure of the water. Boney fish tolerate this treatment very well, in fact, once the water become hyposaline, you will likely note a significant improvement in your fish health and appetite. There is no reason to fear this treatment. You can do a water-change out, in small increments every half hr or so.
Your goal is to drop the specific gravity to 1.009 [48-hrs] gradually. Once all signs of the parasite are gone, then keep your fish in this hypo-saline water for 3-4 wks. If all is well, then you can gradually [over the course of 4-5 days] bring the specific gravity [salinity] back up to normal levels . If all is well after a week, then return the fish to their main tank.
When the fish are eating, offer them quality and varied food soaked in garlic, zoe/zoecon, vitamin C.
