High nitrite levels can wreak havoc on a saltwater tank, causing your fish to struggle for their lives even if there is plenty of oxygen in the water. It's not a common problem in well-established and regularly maintained aquariums, but can happen as a result of too many sudden additions to the population. High levels are a frequent problem in new tanks.
Nitrites are the result of beneficial bacteria breaking down ammonia in the aquarium water. They are the first stage of ammonia break-down, and in ideal conditions the nitrites are immediately further broken down into nitrates. While ammonia is the most harmful to fish, nitrites are less so, and nitrates have an even milder effect. In large quantities, however, nitrites can cause serious health problems and even death in saltwater fish.
The nitrite level in a healthy saltwater tank should be zero. This indicates that ammonia is being broken down rapidly and efficiently. If fish are being overfed, there are too many fish for the amount of water, the temperature is too high or the tank is new and the water is still cycling -- becoming balanced -- nitrite levels can rise to dangerous levels. The nitrites bind to red blood cells in the fish's body and prevent the proper absorption of oxygen. This is commonly referred to as brown blood disease, since the edges of the gills start to turn an unsightly shade of brown.
Symptoms of brown blood disease, or nitrite poisoning, include clamped fins, excessive resting at the bottom of the tank, gulping for air at the surface of the water and swimming near water outlets. Some fish may also rub themselves against objects near the bottom of the tank. Depending on the species, some saltwater fish can tolerate higher nitrite levels than their freshwater counterparts, and they may show only mild symptoms or none at all, but the end result is the same -- the inability to use oxygen present in the water due to the presence of nitrites.
Treatment and Prevention
High nitrate levels require immediate action so that your saltwater fish can get the oxygen they need to survive. A partial water change of about 15 to 20 percent of the water can lower nitrite levels quickly. Skip one day of feeding to reduce waste products and check the filter and aeration systems to make sure they're working properly. If you recently added new fish, you may need to remove them since the tank may not be able to handle the extra waste. Once the nitrite level has dropped back to zero, maintain it by feeding only as much as fish will consume in a few minutes, performing weekly water changes and properly maintaining the tank filter.