I've not posted the general instructions for a hyposalinity treatment before now. Mostly it isn't that difficult to perform. However, there are some pitfalls and for some aquarists (especially those that think it will be easy) it isn't easy. 
Hyposalinity is the nicest (to the fish) and easiest (on the fish) way to treat marine fish of Marine Ich (Cryptocaryon irritans).
Before you treat Marine Ich, please read this: Marine Ich - Myths and Facts
The hyposalinity treatment only treats a very limited number of ciliated parasites. The most notable in this group is Marine Ich (Cryptocaryon irritans). The home aquarist will not be using hyposalinity to treat any other disease or condition. Is this clear? I'll write it again -- hyposalinity cures ONLY Marine Ich.
A hyposalinity treatment will kill: Pods, snails, crabs, invertebrates, corals, live rock, most marine algae, and Marine Ich. This is why it is best performed in a separate, bare bottom, hospital tank. It has been performed successfully in fish only aquariums where there is no live rock and the substrate doesn't have worms and pods in it. But the best treatment tank is a bare bottom hospital tank, set up like a quarantine tank. See here for quarantine information: A Quarantine Procedure
A hyposalinity treatment will not kill other parasites or conditions. A hyposalinity treatment does not kill Marine Velvet (Amyloodinium ocellatum), bacterial infections, injuries, intestinal parasites, external parasites, and a few hundred other diseases. Let's get to it!
GENERAL
Setup/System
Use a suitably clean tank or a setup/establish quarantine tank. The size should 'fit' the fish to be treated. About 5 gallons per inch of fish works out okay for this treatment, except for large adult Angelfishes, Tangs, and Rabbitfishes. For them, it would be better to use the ratio of 9 gallons per inch.
Lighting on the treatment tank should be dimmed so that you can see the fish, but not bright. Provide the fish with some hiding place(s) (e.g. PVC pipe, fake rock, etc.). Do not put substrate, live sand, live rock or any other kind of living thing in the treatment tank. A simple bare tank with a corner sponge filter is more than adequate. Use one corner filter for every 15 gallons of tank water.
Don't add/use power heads or strong circulating pumps. Set the tank up in a quiet area of the home, where there is minimal human traffic. Use heater or chiller and thermometer to hold temperature steady and constant. A UV may be used if you have one suitable for that size tank. Just don't overcook the water! Do not attach a skimmer -- they don't usually work well with this kind of water. Besides, you should be making water changes to control organics/dissolved proteins.
DO NOT USE AN ELEVATED TEMPERATURE during treatment. DO NOT ADD ANY OTHER MEDICATIONS TO HYPOSALINE SALTWATER without knowing it is approved by the medicine manufacturer for use specifically during a hyposalinity treatment. When fish are in a hyposaline liquid, the effects of medicines and medications changes. Some meds become lethal, like copper. NEVER USE COPPER when doing the hyposalinity treatment.
The only different equipment needed besides the above is a refractometer and pH meter. They are less than $50 and $80 respectively and well worth the investment. A hydrometer is just not accurate enough for controlling the salt content in the treatment tank. A pH test kit showing colors just isn't good enough to give you the pH control you need. Control will be essential to a successful treatment.
First Water and Water Changes
Water is taken from the display tank system or water is made up to match the water the fish is coming from. Match specific gravity, temperature and especially pH to very close to the water the fish is currently in, if water is being prepared from salt. The pH should be within 0.05 pH units -- that's how close I mean by 'close.' Get details here: How to Make a Successful Water Change
pH Control
pH is hard to control in a hyposaline solution because at this dilution, the buffer ability of the diluted saltwater is not good. Be prepared for this.
Make small pH + adjustments with fresh pure baking soda (e.g., Arm & Hammer) you find in the grocery store. This is sodium bicarbonate, but I would recommend you use sodium carbonate. Sodium carbonate can be made in the home oven. Take a pound of fresh pure baking soda and spread it out evenly on a large cookie sheet. Put into a preheated oven to 350F. Bake it for 30 minutes. Take the sheet out of the oven, let cool to warm and put into an air-tight, clean container for use. Now the baking soda has been turned into sodium carbonate, a more potent pH + additive. Add the pH + adjuster to water in a cup. Dissolve thoroughly then drip into the QT/hospital tank slowly. Check the pH with a pH meter. No one can say how much to add since it depends on the volume of water, strength of the remaining buffering ability of the water, alkalinity, etc., etc. Just use a little bit and check its affect to get a feel for how much you need to add. It is good to put some pH + adjuster in the water to compensate for evaporation if it is hard to keep the pH up in the right range.
DO NOT try to control the pH with pH buffer or other off the shelf additives. Use only the two mentioned above or a specific strong pH+ control (like sodium hydroxide liquid or pellets).
Do not add the baking soda or sodium carbonate directly to the hospital/quarantine tank. Always thoroughly dissolve some powder in RO/DI or distilled water then drip/add that in slowly to move the pH up.
If you mess up and the pH has lowered considerably (more than 0.2 pH units) then raise the pH VERY SLOWLY -- no more than 0.10 pH units per day. A large pH change can seriously harm a fish, especially a sick one.
PROCESS
Lowering Salinity
Over a period of about 36-48 hours (use 48 for most Tangs, Butterflyfishes, Lionfishes, Puffers, and Dwarf Angelfishes) lower the specific through water removal and RO/DI or distilled water additions. Watch pH and temperature of the added water -- match that of the water being replaced. Use only a refractometer to measure the specific gravity. Lower the specific gravity to a reading of 1.008 to 1.009 sp. gr. units. Hold it there throughout the treatment.
Maintenance
This is why I mentioned above this treatment is not easy on the aquarist.
If the treatment tank has an active biological filter, don't assume it's working. When salinity is lowered the bacteria sometimes enter into a state of suspension and hold off in their metabolism of ammonia and nitrites. Controlling water quality and especially pH will be the challenge. Check for ammonia, nitrites, and pH twice each day at the start, until readings are zero for ammonia and nitrites. Don't count on the pH to remain steady. It must be checked no less than twice a day and if needed, adjusted.
Make water changes to control organics, ammonia, nitrites, nitrates, etc. (No skimmer, carbon, or water treatments are necessary during this treatment. A skimmer will not operate properly in a hyposaline solution.) Even if water quality is good and steady, still make water changes of 35% or more every other day. After the first water used to start the QT, the only water to use to make changes is made up water from salt. DO NOT USE DISPLAY TANK WATER to make water changes in a treatment tank, no matter what disease is being treated. See: How to Make a Successful Water Change.
Nutrition
The fish must be offered and gotten to eat throughout the process. The best foods must be given. Choose the right foods and feed frequently as recommended here: Fish Health Through Proper Nutrition
I would also seriously recommend the fish foods be fortified/supplemented with immune boosters during this time of disease treatment. For suggestions on immune boosters, see: Immune Boosters
Salinity Control
If the tank water salinity is allowed to go up, the Marine Ich parasite will not be killed or stressed into submission. If the tank water salinity goes below the target salinity, the fish are in danger. HOLD THE SPECIFIC GRAVITY very closely within 1.008 to 1.009 sp. gr. units.
Treatment Time
The fish is kept in hyposalinity 4 weeks after the last spot is seen. After the last spot disappears, the hyposalinity continues for another 4 weeks. If during that 4 weeks, if any spot is seen, the time/clock starts over. There must be a minimum of 4 solid weeks with no spot EVER seen.
Raising Salinity
Now raise the specific gravity slowly. The raising of the specific gravity is very stressful on fish and this part must be done slowly. It should take 6 to 7 days to return the water to its normal salinity. Raise the specific gravity by about 0.003 sp. gr. units or less, per day. Less is okay. No need to be ultra conservative and go beyond 8 days to do this. BUT in no way try to shorten the time to less than 6 days.
When the treatment time is over, there is no need to add back distilled water to compensate for water evaporated. Let that be part of the raising of the salinity. When you go to raise the salinity, add small quantities of high specific gravity mixed and aged salt water.
Continue to monitor all water parameters and chemistries.
PROOF
Verification
After the salinity is returned to normal, hold the fish in the treatment tank for another 4 weeks to verify it is cured. Look for spots every day, very closely. Observe the fish behavior, breathing rate, flashing (scratching) and look for any other Marine Ich symptoms. At the end of this time, the fish is cured/free of Marine Ich IF no other symptoms are seen/observed.
Hyposalinity is the nicest (to the fish) and easiest (on the fish) way to treat marine fish of Marine Ich (Cryptocaryon irritans).Before you treat Marine Ich, please read this: Marine Ich - Myths and Facts
The hyposalinity treatment only treats a very limited number of ciliated parasites. The most notable in this group is Marine Ich (Cryptocaryon irritans). The home aquarist will not be using hyposalinity to treat any other disease or condition. Is this clear? I'll write it again -- hyposalinity cures ONLY Marine Ich.
A hyposalinity treatment will kill: Pods, snails, crabs, invertebrates, corals, live rock, most marine algae, and Marine Ich. This is why it is best performed in a separate, bare bottom, hospital tank. It has been performed successfully in fish only aquariums where there is no live rock and the substrate doesn't have worms and pods in it. But the best treatment tank is a bare bottom hospital tank, set up like a quarantine tank. See here for quarantine information: A Quarantine Procedure
A hyposalinity treatment will not kill other parasites or conditions. A hyposalinity treatment does not kill Marine Velvet (Amyloodinium ocellatum), bacterial infections, injuries, intestinal parasites, external parasites, and a few hundred other diseases. Let's get to it!
GENERAL
Setup/System
Use a suitably clean tank or a setup/establish quarantine tank. The size should 'fit' the fish to be treated. About 5 gallons per inch of fish works out okay for this treatment, except for large adult Angelfishes, Tangs, and Rabbitfishes. For them, it would be better to use the ratio of 9 gallons per inch.
Lighting on the treatment tank should be dimmed so that you can see the fish, but not bright. Provide the fish with some hiding place(s) (e.g. PVC pipe, fake rock, etc.). Do not put substrate, live sand, live rock or any other kind of living thing in the treatment tank. A simple bare tank with a corner sponge filter is more than adequate. Use one corner filter for every 15 gallons of tank water.
Don't add/use power heads or strong circulating pumps. Set the tank up in a quiet area of the home, where there is minimal human traffic. Use heater or chiller and thermometer to hold temperature steady and constant. A UV may be used if you have one suitable for that size tank. Just don't overcook the water!
Do not attach a skimmer -- they don't usually work well with this kind of water. Besides, you should be making water changes to control organics/dissolved proteins.DO NOT USE AN ELEVATED TEMPERATURE during treatment. DO NOT ADD ANY OTHER MEDICATIONS TO HYPOSALINE SALTWATER without knowing it is approved by the medicine manufacturer for use specifically during a hyposalinity treatment. When fish are in a hyposaline liquid, the effects of medicines and medications changes. Some meds become lethal, like copper. NEVER USE COPPER when doing the hyposalinity treatment.
The only different equipment needed besides the above is a refractometer and pH meter. They are less than $50 and $80 respectively and well worth the investment. A hydrometer is just not accurate enough for controlling the salt content in the treatment tank. A pH test kit showing colors just isn't good enough to give you the pH control you need. Control will be essential to a successful treatment.
First Water and Water Changes
Water is taken from the display tank system or water is made up to match the water the fish is coming from. Match specific gravity, temperature and especially pH to very close to the water the fish is currently in, if water is being prepared from salt. The pH should be within 0.05 pH units -- that's how close I mean by 'close.' Get details here: How to Make a Successful Water Change
pH Control
pH is hard to control in a hyposaline solution because at this dilution, the buffer ability of the diluted saltwater is not good. Be prepared for this.
Make small pH + adjustments with fresh pure baking soda (e.g., Arm & Hammer) you find in the grocery store. This is sodium bicarbonate, but I would recommend you use sodium carbonate. Sodium carbonate can be made in the home oven. Take a pound of fresh pure baking soda and spread it out evenly on a large cookie sheet. Put into a preheated oven to 350F. Bake it for 30 minutes. Take the sheet out of the oven, let cool to warm and put into an air-tight, clean container for use. Now the baking soda has been turned into sodium carbonate, a more potent pH + additive. Add the pH + adjuster to water in a cup. Dissolve thoroughly then drip into the QT/hospital tank slowly. Check the pH with a pH meter. No one can say how much to add since it depends on the volume of water, strength of the remaining buffering ability of the water, alkalinity, etc., etc. Just use a little bit and check its affect to get a feel for how much you need to add. It is good to put some pH + adjuster in the water to compensate for evaporation if it is hard to keep the pH up in the right range.
DO NOT try to control the pH with pH buffer or other off the shelf additives. Use only the two mentioned above or a specific strong pH+ control (like sodium hydroxide liquid or pellets).
Do not add the baking soda or sodium carbonate directly to the hospital/quarantine tank. Always thoroughly dissolve some powder in RO/DI or distilled water then drip/add that in slowly to move the pH up.
If you mess up and the pH has lowered considerably (more than 0.2 pH units) then raise the pH VERY SLOWLY -- no more than 0.10 pH units per day. A large pH change can seriously harm a fish, especially a sick one.
PROCESS
Lowering Salinity
Over a period of about 36-48 hours (use 48 for most Tangs, Butterflyfishes, Lionfishes, Puffers, and Dwarf Angelfishes) lower the specific through water removal and RO/DI or distilled water additions. Watch pH and temperature of the added water -- match that of the water being replaced. Use only a refractometer to measure the specific gravity. Lower the specific gravity to a reading of 1.008 to 1.009 sp. gr. units. Hold it there throughout the treatment.
Maintenance
This is why I mentioned above this treatment is not easy on the aquarist.
If the treatment tank has an active biological filter, don't assume it's working. When salinity is lowered the bacteria sometimes enter into a state of suspension and hold off in their metabolism of ammonia and nitrites. Controlling water quality and especially pH will be the challenge. Check for ammonia, nitrites, and pH twice each day at the start, until readings are zero for ammonia and nitrites. Don't count on the pH to remain steady. It must be checked no less than twice a day and if needed, adjusted.
Make water changes to control organics, ammonia, nitrites, nitrates, etc. (No skimmer, carbon, or water treatments are necessary during this treatment. A skimmer will not operate properly in a hyposaline solution.) Even if water quality is good and steady, still make water changes of 35% or more every other day. After the first water used to start the QT, the only water to use to make changes is made up water from salt. DO NOT USE DISPLAY TANK WATER to make water changes in a treatment tank, no matter what disease is being treated. See: How to Make a Successful Water Change.
Nutrition
The fish must be offered and gotten to eat throughout the process. The best foods must be given. Choose the right foods and feed frequently as recommended here: Fish Health Through Proper Nutrition
I would also seriously recommend the fish foods be fortified/supplemented with immune boosters during this time of disease treatment. For suggestions on immune boosters, see: Immune Boosters
Salinity Control
If the tank water salinity is allowed to go up, the Marine Ich parasite will not be killed or stressed into submission. If the tank water salinity goes below the target salinity, the fish are in danger. HOLD THE SPECIFIC GRAVITY very closely within 1.008 to 1.009 sp. gr. units.
Treatment Time
The fish is kept in hyposalinity 4 weeks after the last spot is seen. After the last spot disappears, the hyposalinity continues for another 4 weeks. If during that 4 weeks, if any spot is seen, the time/clock starts over. There must be a minimum of 4 solid weeks with no spot EVER seen.
Raising Salinity
Now raise the specific gravity slowly. The raising of the specific gravity is very stressful on fish and this part must be done slowly. It should take 6 to 7 days to return the water to its normal salinity. Raise the specific gravity by about 0.003 sp. gr. units or less, per day. Less is okay. No need to be ultra conservative and go beyond 8 days to do this. BUT in no way try to shorten the time to less than 6 days.
When the treatment time is over, there is no need to add back distilled water to compensate for water evaporated. Let that be part of the raising of the salinity. When you go to raise the salinity, add small quantities of high specific gravity mixed and aged salt water.
Continue to monitor all water parameters and chemistries.
PROOF
Verification
After the salinity is returned to normal, hold the fish in the treatment tank for another 4 weeks to verify it is cured. Look for spots every day, very closely. Observe the fish behavior, breathing rate, flashing (scratching) and look for any other Marine Ich symptoms. At the end of this time, the fish is cured/free of Marine Ich IF no other symptoms are seen/observed.
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