Nice thread !!! 
Hyposalinity
Jay Hemdal 2021
This term refers to a long-term bath treatment for marine fishes in which the salt content of the water is lowered and held below the point that certain parasites can survive. The salt level reached, and the time of the treatment are the two variables. Hyposalinity can be used as a primary treatment against certain marine fish diseases (see below) or as a component of an active quarantine for newly acquired fish.
To clarify for beginners: you cannot perform hyposalinity on fish when invertebrates are in the tank. You must isolate the fish to a fish-only situation.
Before instituting a hyposalinity treatment, you need to decide what measurement units you plan on using – salinity or specific gravity. Then, you must have a very accurate measuring device. A laboratory grade hydrometer is best for accurate specific gravity measurements, or to standardize another measurement means. The device employed must be capable of accurately measuring down to one part per thousand, or one specific gravity unit. Better yet is a device that can read to twice that resolution. For example; capable of measuring 16.5 ppt or 1.0125 specific gravity units. Most handheld refractometers are not accurate enough. Swing arm hydrometers can measure this small of a difference, but they must be first checked for accuracy against a laboratory hydrometer.
There are two basic means to administer a hyposalinity treatment: in the display tank or in a quarantine/treatment tank. Invertebrates and some algae cannot survive hyposalinity, so must be removed prior to a hyposalinity treatment (of course, they then need to be cleared of potential disease by use of a proper fallow period). There are benefits and drawbacks to administering hyposalinity in these two modes:
Quarantine tank: Little to no microbiome die-off. Less salt lost. Ability to add other medications during the process if required. System may not be biologically robust enough to manage ammonia levels. Poor pH buffering. Requires a second aquarium and letting the infected display tank go fallow. Ability to use dechlorinated tap water to dilute the seawater, saving time and cost over using reverse osmosis water.
Display tank: Reduces the fallow period and doesn’t require a second tank, unless invertebrates are present. Better pH control in most instances. Usually, more saltwater is lost during the process due to the larger aquarium. In established tanks with much microfauna, there may be a significant die-off of algae and small invertebrates. May need to use reverse osmosis water as the dilutant in order to reduce nutrient loading for algae control.
Hyposalinity is really a targeted treatment for marine ich (Cryptocaryon irritans), marine flukes (including Neobenedenia sp.) and has some control over Turbellarian worms (including “black ich”). It does NOT control velvet (Amyloodinium marinum) or clownfish disease, Brooklynella hostilis. There are reports that hyposalinity actually increases the incidence of Uronema, especially in newly acquired fish. Hyposalinity has no direct affect on bacterial, fungal or any internal infections of marine fishes. In rare cases, fish may be infected with brackish water flukes as they travel through the supply chain. In these rare instances, hyposalinity will actually make the disease worse. Experiments have been tried in these cases with “hypersalinity”, holding the fish at higher-than-normal salinity, but results have been poor. In the case of brackish water flukes, praziquantel should be used.
Begin the hyposalinity treatment by determining the final salinity as well as the time and schedule to reach that level. Full hyposalinity is required in order to treat Cryptocaryon, and that equates to a specific gravity of 1.009 at 75 degrees F. or a salinity of 12 parts per thousand. If only Neobenedenia is being controlled, the target can be set a bit higher, to a specific gravity of 1.012 or a salinity of 16 ppt. The time required to reach these endpoints depends on the starting salinity of the tank, as well as the severity of any disease. If severe disease is present, dropping the salinity more quickly, while stressful, starts the treatment faster. Likewise, starting a tank into hyposalinity from a specific gravity of 1.026 will require more time than if the aquarium is already at a lower specific gravity, perhaps 1.022. There is NO firm rule as to how fast to lower the salinity, and some fish simply do not handle hyposalinity as well as others. Inshore fishes tend to handle hyposalinity much better than do deep reef fishes. A general rule of thumb is to reduce the salinity of the aquarium to hyposalinity over 48 hours, in at least four steps. In cases where there is severe disease, hyposalinity should be reached in 24 hours using four steps. Deep reef fish, showing no overt disease and being held at a salinity of 35 ppt might be best lowered in six steps, over 72 hours. It is vital to understand that marine fish can handle a reduction in salinity very well, but the return to normal salinity at the end of the treatment must be done much more slowly. The general recommendation is to return the fish to normal salinity over five days, in at least ten increments. Smaller fish seem more stressed by a rise in salinity than do larger fish. This is likely due to vastly different surface to volume ratios between large and small fish.
During hyposalinity, always monitor the ammonia level as well as the pH to ensure they remain in the proper range. Don’t try to maintain full reef aquarium pH levels during hyposalinity – a slightly lower pH reduces the toxic affect of any ammonia. As long as the aquarium is well aerated, a pH above 7.6 is adequate. Don't raise the water temperature beyond normal. That is a treatment for freshwater ich, not marine ich. 79 degree F. is a good target temperature.
Exactly how to make the salinity adjustments in an accurate fashion requires some mathematical calculations. You need to decide how many days the transition will take, and then how many adjustments you are able to make each day. You then know the number of dilutions you will be making. After that, you need to calculate the change in salinity (in ppt or specific gravity units). You then divide the change in salinity by the number of changes to be made, and that gives the magnitude of each change. However, you really need to recalculate for each change after the first one. Here is an example:
A 20-gallon aquarium at 35 ppt needs to be brought down to 16 ppt over three days. Four adjustments will be made each day, for a total of 12. The change in salinity totals 16 ppt. To make this change in 12 steps, each drop should be 1.33 ppt. Multiply 1.33 by 100 and divide by 35. That equals about 4%. Removing 4% of the tank water and replacing it with freshwater will drop the salinity to 33.7. Repeat the process for each of the next 11 steps. Raising the salinity works in reverse.
When it is all said and done, most people just eyeball the changes as the math becomes pretty convoluted. Raising the salinity can be pretty wasteful if you drain water out of the tank and replace it with slightly more saline water. Many people end up making strong brine solutions and mix that in slowly. Beware though; if you dissolve sea salt at a salinity higher than about 50 ppt, subtle changes can occur, including precipitation of carbonates.
Fish should remain in hyposalinity for 30 days beyond the date that obvious disease symptoms were last seen. Neobenedenia treatments are best to run for 35 days.
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