How do you treat?
It is very important that you can accurately measure the salinity or specific gravity of the water. Cheap hydrometers, especially the swinging arm variety, do not have enough accuracy. If the salinity is too low, it is possible the health of the fish will be compromised. If the salinity is too high, it may have no affect on the parasites, as discussed above. A refractometer is the safest method for measuring salinity. If you don't have access to a refractometer, a lab grade floating glass hydrometer should suffice.
The goal is to granularly reduce the salinity of the water to between
12 and 14ppt and leave it at that salinity for
at least 4 weeks but preferably 6 weeks. Basically continue the treatment for at least 4 weeks after the last spots disappeared.
The salinity must be lowered gradually to give the fish time to adjust to the lower salinity and more importantly ensures the bacteria in the biological filter can adjust.
It should take around 2 days to get from 35ppt to 14ppt.
Your starting point should be between 1.025 and 1.027. Replace about one fifth of the volume with RO, RO/DI or aged freshwater that has been well aerated. Repeat this 12, 24 and 36 hours later, monitoring the specific gravity along the way. After the fourth water change the specific gravity should be 1.010 or pretty close. Wait a few hours to make the final adjustment to get down to 1.009. Note that you can estimate the resulting specific gravity. If you are changing one fifth of the water and the current specific gravity is 1.025 the result will be:
((1.025*4)+1.000)/5 = 1.020 approx.
Then, after 12 hours:
((1.020*4)+1.000)/5 = 1.016 approx.
After 24 hours:
((1.016*4)+1.000)/5 = 1.013 approx.
After the 4th change:
((1.013*4)+1.000)/5 = 1.010 approx.
Water temperature influences specific gravity and if you heat water without changing the salinity the specific gravity will decrease. As the goal is to keep the salinity between 12 and 14ppt it is important to know the temperature as well as the specific gravity.
What temperature should you use? Some people have argued that raising the temperature is good because it speeds up the life cycle of the parasite. While this is true, the elevated temperature also raises the metabolic rate of the fish causing increased oxygen and energy consumption and somewhat negating the benefits of the hyposalinity. Temperatures close to "normal", those the fish are used to, will be the best. Ensure the temperature is taken into consideration for the specific gravity.
While the fish are being treated it is extremely important to closely monitor the pH and specific gravity. Unless you are treating in a well established tank with an established biological filter
there will be a tendency for the pH to drop and this must be monitored. This can also happen in an establish tank due to the lower salinity. If the pH starts to drop, water should be changed or buffer very carefully added to the system. If the fish are sick, too rapid changes in pH will not be beneficial.
The specific gravity also must be monitored as
excessive evaporation will cause the salinity to rise and possibly create suitable conditions for the free swimming parasite. If the salinity does rise, it may be necessary to extend the length of the treatment after the salinity has been lowered again.
Hyposalinity should be maintained for at least 4 weeks but 6 weeks is preferable. If there is any reinfection of the "Ich" during the treatment, the treatment should be extended to at least 4 weeks after the disappearance of the last cyst.
When the treatment is complete, the salinity should be raised gradually to normal over a number of days. Water changes with normal or even high salinity water is the easiest way to get the salinity up. Calculations using averages, as before, can be used to determine the appropriate specific gravity of the replacement water. If you replace one sixth of the 1.009 water with water at 1.025, the specific gravity will be raised to 1.012:
((1.009*5)+1.025)/6 = 1.012 approx.
As the salinity approaches normal it will be necessary to either replace more water in each change or use water with a higher than normal salinity. If you have been treating in a quarantine tank, you will need to leave the display tank with no fish in it for at least 30 days. This will ensures that all remaining parasites in the tank have gone through their full life cycle and the infective forms have died.
If you have been treating in the display tank, you should wait some time before returning any invertebrates or live rock to ensure that the biological filter has had time to adjust to the salinity and load.