[NaH2O]

DrHank - hyposalinity is a very effective treatment for ich, and can also be used in QT when introducing new livestock. Fish under stress can have osmoregularity issues. Hyposalinity can decrease stress by allowing the fish to save energy normally used in osmoregulation. Provided pH is maintained along with other parameters such as temperature, ammonia, nitrites, and nitrates in a QT, I think it is a great procedure.

Here is some more information on osmoregulation:

Osmoregulation: Please give me a drink!

Marine fish (teleosts) have the exact opposite problem to that encountered by freshwater teleosts. Their body fluids are, again, 1/3 of that of sea water but this time they are in sea water so their body fluids are hypoosmotic to their environment. As a result they will tend to lose water by osmosis to the environment through their skin but mostly through their gills. Consequently, they have developed mechanisms and behaviour to compensate for this water loss. Firstly, the kidneys of marine teleosts are modified in such a way that very little water is extracted from the blood, some species even lack certain kidney structures and can't eliminate water (Gordon, 1977; Moyle and Cech, 1982). This results in a reduction in the loss of water by the production of urine. However, water is still being lost by the gills and this cannot be stopped, so the only method left is to somehow replace the water as quickly as it is lost. Marine teleosts accomplish this by actually drinking water, the most reliable drinking rates reported in the literature range from 3-10 ml/(kg hr) (Gordon, 1977). However, drinking water by itself cannot solve the problem, a complex series of events must first occur in the digestive tract. These events are not yet well understood but it is known that most of the water is absorbed as are the monovalent ions Na+ and Cl- (they are, after all, drinking salt water!), while the divalent ions (such as magnesium and sulfates) are excreted by the kidneys (Gordon, 1977). Sodium (Na+) and chloride (Cl-) also move by diffusion into the body through the gills. Therefore, Na+ and Cl- ions will accumulate in the body of the fish and must be eliminated, this is accomplished by special cells in the gills called chloride cells, which me these ions out of the body by active transport (Moyle and Cech, 1982; Gordon, 1977).

From the above information some practical tips for the hobbyist can be gained. Since marine fish must constantly expel various solutes, such as sodium and chloride ions, against an osmotic gradient, a great deal of energy is required. Therefore, anything that you can do to lower the osmotic gradient will benefit the fish in terms of energy expenditure. The simplest way of doing this is to lower the salinity of the water as much as possible, particularly for a fish in distress (i.e. diseased). This alone can sometimes be enough to ease their burden. Of course any such change must be extremely gradual and must not get to the point where the fish is in obvious stress. Another problem comes when invertebrates are added, especially the soft-bodied ones such as anemones and corals; a drop in salinity can be disastrous for them. Since marine fish produce very concentrated urine, their waste products can pollute a tank far quicker than a freshwater fish which produces much more dilute wastes. That is why you can usually put in many more freshwater fish than marine fish in the same volume of water. That is why paying attention to the water quality of a marine tank is so much more critical than in a freshwater tank. With the advent of dry/wet filter systems from Europe, the load in marine aquaria can now be greatly increased due to the superior ability of the filter to handle waste products. That is why the so called "mini-reef" systems are becoming so popular with hobbyists, many more animals can be kept in a smaller volume of water with little risk of pollution.

From: Quarantine Tank Methodology - Part 2

Lowering the salinity to less than that of natural seawater is a common practice among public aquariums, hatcheries and fisheries. Lessening the gradient between the internal fluids of the fish and their surrounding environment reduces osmotic pressure. This conserves metabolic energy that would normally be required for osmoregulation. Since hydromineral imbalances are inherent to stress in fish, lowering the salinity can reduce the effects of these stressors. Dr. Cheung (Cheung et al., 1979) reported that boney marine fish could be kept in a salinity of 16ppt “indefinitely.” I do not suggest maintaining marine fish in a salinity less than natural seawater on a permanent basis.

From: Reducing Losses Associated with Transport and Handling in Marine Aquarium Fish

Significant portions of post shipment losses are due to osmoregulatory dysfunction and stress-mediated diseases occurring within the first week after transport (Johnson & Metcalf, 1982. Carmicheal et. al, 1984). Stress in fish causes osmoregulatory dysfunction (Harrell & Moline, 1992. Weirich et. al, 1992). This can lead to mortalities (Tomasso et. al, 1980). Reducing the gradient (difference in salinity) between the internal fluids of fish and the surrounding ambient water alleviates water and ion disturbance ((Wedemeyer, 1996). Manipulating the salinity of the transport water upward for freshwater fish and conversely downward for saltwater fish is effective for controlling osmoregulatory disturbances and reducing losses (Carneiro &Urbinati, 2001). Fish held in water that is close to isotonic (the salinity of the surrounding ambient water is close to the internal fluids of the fish) have increased stress resistance (Lim et. al, 2000). These fish also display a significantly lower mortality rate at 7 days post shipment.

From: Quarantine of Marine Fish (Teleost) Using Hyposalinity

The other advantage that is of great interest with hyposalinity, is the reduction of osmotic stress on fish with abrasions or lesions. In theory, the reduction of the osmolar gradient between the internal tissues and the surrounding environment would be beneficial to injured mucus and epidermal tissue. This reduction in the osmolar gradient, in theory, greatly reduces the loss of water from the fish to the surrounding environment. The ability to maintain hydration in an injured marine fish too small to administer fluids could prove very beneficial. Many more studies, which are ongoing at this time, and sample collections remain to prove this theorized aspect of low salinity.

bluespotjawfish - Have you seen any feces output at all from your tang? There was a thread on another forum about a Naso with a similar problem (I'm not sure if I can link to the thread here?). I'll give a quote when I asked about the large lumpiness in the gut area of the tang:

"How can you differentiate between the lumpiness being a problem/infection, and one of food in the gut."

In new fish or fish you have never kept it can be difficult but in general it's more a matter of being familiar with the fishs' common appearance. With food, there will be small nodular bulges/lumps as the gut fills up, infections will be more rounded and usually pronounces in one or more area's as apposed to uniform "lumpiness" from eating. It's also knowing how much food is fed vs the size of the inflamation. If the belly area keeps getting larger and the food intake doesn't support it, there is a strong possibility of that or intestinal blockage.

BTW - the tang from the above quote was treated with KanaPlex in hyposaline conditions and made a recovery.

As for your pH issues, you will want to get that back up. Boomer has recommended in hyposalinity to use a product called: SeaChem Labs Marine Buffer (8.3). He recommends adding 1/2 according to the instructions and then check pH. According to Boomer, the pH will stay at 8.3, once you get it there using this buffer. BUT, don't think of this as a normal buffer. In a hyposaline QT tank it's Ok to use.

Lastly, when you do go to raise your salinity, do so in a slooow manner. Over the course of the week. Fish can withstand rapid drops in salinity, but cannot take rapid increases in salinity.

Hope this helps.