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[ GO FISHNET ] |
Volume 16, Fall 1999
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[ GO FISHNET ] |
Nitrite Toxicity in Marine Aquariums
by Thomas Frakes and Bob Studt
For many years we have heard how toxic nitrite is to fish. Although this is certainly true for freshwater aquarium fish, authors of marine aquarium guides have stated the same toxicity levels for marine aquariums. Usually the recommendation has been to keep the nitrites below 0.1 milligrams per litre (mg/l), approximately 0.1 parts per million (ppm). The nitrite level in a properly functioning aquarium is normally below this.
In spite of this 0.1 mg/l recommendation found in almost all marine handbooks, the toxicity of nitrite in saltwater is less than commonly believed. Numerous studies dating back to the 1970's have shown a dramatic decrease in the toxicity of nitrite when salt or sodium chloride was added to aquarium water. The mechanism responsible for this appears to take place at the gills, where the chloride ion interferes with the absorption of the nitrite ion. Spotte (1992) lists many papers, most from studies conducted on food fish, that document much higher median lethal concentrations for nitrite, some as high as 500 mg/l.
We conducted a limited test in an effort to show that nitrite toxicity to reef fish is less than it was previously assumed to be. We used tank-raised clownfish (Amphiprion ocellaris) supplied by Joe Lichtenbert to Reef Propagations. Twenty 1 1/4 inch long juvenile fish were randomly divided into four groups of 5 fish each, and each group was placed in a newly set up 10 gallon aquarium. Each aquarium was filled with 8.5 gallons of Instant Ocean® sea water mixed to 30 parts per thousand (ptt) salinity at a temperature of 78º F. After a 24 hour adjustment period, a stock solution of sodium nitrite was added to each tank to achieve the test levels of 0 mg/l (the control), 1.0 mg/l, 10 mg/l, and 100 mg/l nitrite-nitrogen (NO2-N). Multiply these levels by 3.3 to obtain results in the nitrite ion (NO2) scale. Tests results were obtained using a spectrophotometer and calibrated faqs TesT reagents. Fish were not fed, and ammonia levels were negligible during the test. Daily testing confirmed that the Nitrite concentration in each tank remained at the target level for the 96 hour duration of the test.
The two higher levels of nitrite produced increasingly labored breathing in the fish. Rapid breathing was noted in all fish at 10 mg/l tank were very lethargic and breathing hard. The only mortalities occurred in the 100 mg/l tank, one after 50 hours and the second after 74 hours. After 96 hours, a 96-98% water exchange was made in all the tanks, including the control. Behavior of the stressed fish returned to normal within 48 hours, and no more deaths occurred over the next 30 day period.
The 40% mortality at the 100 mg/l level suggests a median lethal concentration (LC50) just over 100 mg/l NO2-N. This is consistent with the 86 mg/l to 187 mg/l range of LC50 results for five marine species listed by Stoskopf (1993). The 10 mg/l level produced stress but no lasting ill effects. At the 1.0 mg/l level no changes in behavior were noted.
During the run-in of a new biological filter it is rare for the NO2-N level to reach 10 mg/l. If this level is reached it generally lasts only a few days before dropping. Thus, it appears that the level of nitrite in an aquarium is less critical to marine fish survival than popular literature would indicate.
Fish do die of "new tank syndrome". This mortality is more likely due to a high level of ammonia than of nitrite. For many marine fish, 50% will die at concentrations of ammonia-nitrogen (NH3-N) as low as 1.8 mg/l (LC50). This level of ammonia is very easy to reach during a new tank cycle. The non-lethal level of 10 mg/l NO2-N is rarely reached.
Freshwater fish and marine fish respond to high levels of nitrite and ammonia in very different ways. Let's compare the nitrogen cycle in freshwater aquarium (pH 7.0) and a marine aquarium (pH 8.2). We will assume an input of 5 mg/l NH3-N before bacteria convert the ammonia to 5 mg/l NO2-N, and finally to 5 mg/l nitrate-nitrogen (NH3-N). The 5 mg/l ammonia level would be deadly to most of the marine fish, but may not affect the freshwater fish. However, the eventual 5 mg/l nitrite would probably kill the freshwater fish, but have little effect on the marine fish. Any deaths of the marine fish during the nitrite peak would probably be due to residual ammonia damage to the gills and to stress related disease.
In freshwater aquariums, adding a half ounce of salt per gallon of water will reduce nitrite toxicity for many fish (Stoskoph 1993). Ina marine aquariums we should shift the emphasis back toward the real killer, ammonia.