ick out break in my 120 gallon

[krowleey]

Risk to the fish is a function both of the lowered specific gravity and time at that lower salinity. The excerpts below support 1.009 as the lower limit. You could probably go lower with careful monitoring of the fish. The first article suggests there is risk of kidney failure over time. An important question is at what salinity can the Cryptocaryon no longer survive, and is there a need to go much lower than that?

From: http://reefkeeping.com/issues/2003-08/sp/index.php

Treatment Option 4 - Hyposalinity:

Low salinity has been demonstrated to be an effective treatment against Cryptocaryon irritans (Noga, 2000). A salt level of 16 ppt or approximately 1.009-1.010 specific gravity at 78-80*F for 14 days was reported to kill the parasite. I have never experienced problems when placing fish into a hyposalinity treatment, but have routinely witnessed fish showing obvious signs of distress when brought back to normal salinity levels too quickly. For that reason, I try to limit the specific gravity increase 0.001-0.002 points per day.

One of the alleged benefits of this treatment is the resulting conservation of energy for the affected fish. Reef fish have to constantly drink saltwater and excrete the salt to maintain the proper osmotic balance. Lowering the salinity of the surrounding environment eases this energy demand on the sick fish, thereby allowing them to expend more energy towards fighting the infection (Kollman, 1998 and Bartelme, 2001). On the contrary, keeping fish in low salinity means that they don't "flush" their kidneys sufficiently. After long-term exposure, this can cause kidney failure and kill the fish (Shimek, pers. comm..)

and from: http://www.wetwebmedia.com/ichartmar.htm

A) Hyposalinity, lowered specific gravity. Some advocates place lower spg. as low as 1.009. This can work if your fishes are not too challenged already or the pathogen too virulent, however it will not effect a permanent system cure. Know that most common measures of specific gravity are temperature specific and that most non-fish livestock will not tolerate the lower limit (14-16 ppt salinity) necessary to kill off the parasites. Therefore your fishes will have to be separated from your non-fish livestock if you're using hyposalinity as a treatment mode. And there are exceptions, variations to consider using hyposalinity. Cartilaginous fishes (sharks, rays) cannot be treated in this fashion... and such osmotic changes need to be made gradually (over days).

As you might expect from its name, hyposalinity or osmotic shock therapy (OST) is a method of disease treatment that simply involves maintaining a marine aquarium at a much lower salinity than normal. For fish only systems, a salinity of 11 ppt (1.008 SG) is recommended for best results (Lowry 2004) when treating protozoan parasites. In aquaria housing delicate invertebrates, such as reef systems, a modified form of OST that uses a somewhat higher salinity around 22-23 ppt (1.017 SG) is often employed to control outbreaks of parasites (Thiel 2003). In general, the salinity most often used for OST and cited in the literature is 16 ppt or a specific gravity of 1.012 (Lowry 2004), which is a good compromise for most systems.

http://www.wetwebmedia.com/ca/volume_4/V4I4/hyposalinity/OST.htm

Do not hesitate to maintain the hyposalinity for the entire treatment period. OST needs to maintained for at least 3 weeks in order to assure that all of the encysted parasites have reached the free-swimming stage of their life cycle and been killed. A 6 week treatment period even more effective and is the recommended treatment protocol. For example, the Oklahoma Aquarium routinely quarantines all of their marine display fish (i.e., teleosts) at a salinity of 11 ppt (1.008 SG) for 30-45 days (Lowry 2004).

http://www.wetwebmedia.com/ca/volume_4/V4I4/hyposalinity/OST.htm

thats just a start, while 1.010 may work for some strains, 1.008 -1.009 will work 100% of the time. there is no evidence of any hyposalinity resistant strains of marine ich i have ever heard of and i looked all over the place on the net. and using copper with hyposalinity is asking for trouble, so i am having doubts about your hydrometer accuracy. It seems people that have had hypo fail for them, missed a step or tried to make their own steps for the process instead of using the basic formula that works.