Do a big water change.
Next, you do need to do one of three things. Adding salt, as your problem is nitrite poisoning, not ammonia at this stage, is one possibility. Salt is the recommended alleviator for brown blood disease which is the common name for nitrite poisoning.
Brown blood disease occurs in fish when water contains high nitrite concentrations. Nitrite enters the bloodstream through the gills and turns the blood to a chocolate-brown color. Hemoglobin, which transports oxygen in the blood, combines with nitrite to form methemoglobin, which is incapable of oxygen transport. Brown blood cannot carry sufficient amounts of oxygen, and affected fish can suffocate despite adequate oxygen concentration in
the water. This accounts for the gasping behavior often observed in fish with brown blood disease, even when oxygen levels are relatively high........
Sodium chloride (common salt, NaCl) is used to “treat” brown blood disease. Calcium chloride can also be used but is typically more expensive. The chloride portion of salt competes with nitrite for absorption through the gills........
from
https
/www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&ved=0CGEQFjAG&url=https%3A%2F%2Fsrac.tamu.edu%2Findex.cfm%2Fevent%2FgetFactSheet%2Fwhichfactsheet%2F110%2F&ei=YcfeUe-hHLa34APjyICgBA&usg=AFQjCNGBzzMg5b9TUwF127-tY8CSSSgSMQ&bvm=bv.48705608,d.dmg
If nitrite is present, a water change should be performed. If the fish species in the tank will tolerate increased salinity, salt can be added to the aquarium to increase the level of chloride in the water. Chloride levels should be at least six times greater than nitrite levels. Chloride prevents the uptake of nitrite by the fish's blood thus eliminating the effects of brown blood disease. However, the addition of salt/chloride does not remove nitrite from the system, and the source of the nitrite must be controlled.
from
http/florida-agriculture.com/consumers/recreation/aquafish/qual_nitrocycle.html
The article specifies that you know the Cl- concentration in your water in order to calculate the amount you have to add. Very few of us keep track of that...
OK True, but I think it's reasonably safe to assume it is zero in most instances. The amount required to achieve 100ppm (100mg/l) Cl- in a 800L (200gl) tank is 100mg/L*800L*1/0.66 (0.66 is the mass percentage that Salt is of Cl-). This works out to about 14g, or one tablespoon. Aquarium salt is typically added in dosages of 40X this much. The reason that article suggests you calculate it that way is they are talking about millions of gallons. A few extra Tonnes of salt is a big added cost, so taking the existing Cl- into account is important.
from a site for which the forum rules do not allow me post a URL- sorry.
But you should be able to calculate how much salt to add for a tank your size. However, 1st you must know the ppm of nitrite in your tank. As you want at least 6 times the ppm of salt as there is nitrite. You will need a test kit for this or have a really savvy stores to help. The common nitrite kits for the hobby do not read high enough and often a diluted test is needed. Use the above formula and change the 800L to the L for your tank. Also 1 mg/L = 1 ppm.
A second possibility is to turn to Prime by SeaChem or Amquel+ by Kordon, both are dechlors which also supposedly detoxify nitrite and nitrate. These will also slow down your cycling, but will likely let it complete very slowly with the fish staying alive.
No matter what, you must take immediate action to help the fish. The 1st and fastest is to change 75% of the water and then decide whether you want to do the salt addition or the chemical alternative. Do not do both at the same time unless you try one of the chemicals and it doesn't appear to do the trick. Then try the salt, or vice versa. Do not wait after the water change to either begin to add salt or to purchase one of the two products.
A third possibility is to find either Tetra Safe Start or Dr Tim's One and Only Nitrifying Bacteria and add them to your tank ASAP. These are live nitrifying bacteria which will go to work pretty fast fixing your problem. If you choose this route,
Do Not Add An Overdose of the two mentioned dechlors. They will work against the bacteria being added. The amount of salt you might also want to use should not be sufficient to effect the nitrifying bacteria. It is important that you do the big water change of 75% before you add a bacterial supplement as high ammonia or nitrite levels can kill them. You can also try to borrow cycled media from local stores or fish keepers, but this will be less reliable in terms of knowing what you are getting compared the the two bottled products. Cycled media may be difficult to find fast.
One last observation, salt is not wanted long term in fresh water tanks. The only way salt is removed from a tank is via water changes.
