7/8/11 Edit - Stickied this to keep the information at the top for the new folks.
We all know what the Nitrogen Cycle is (for those of you who don't, click here
; take notes!), but rarely does anyone explain why the components of the nitrogen cycle (ammonia/nitrite/nitrate) are harmful to fish. The ongoing and persistent explanation is "Ammonia and nitrite bad, nitrate less bad, because we said it is", and while this certainly is true, I thought it might help some of the newer folk to explain exactly what each does to your fish. I'll try to keep it as simple as I can while still presenting all the information needed; I know a lot of you probably don't want to learn biochemistry just to keep an aquarium, but this is important stuff.
To start off, what is
this ammonia business we keep hearing about? Ammonia is a compound of nitrogen and hydrogen. Although ammonia often gets a bad rap through our hobby, it's actually a critical part of our ecosystem, as it forms an essential link in the food chain by being food for plants, which feed the cows, which feed the people...you get the point. That's not to say that ammonia isn't bad stuff, however. It's caustic, meaning it has the ability to corrode matter; you can see how this can be a problem for living organisms. In humans and other terrestrial animals, ammonia is converted into a phosphate by an enzyme in our body, and is then put into our urea cycle to either by recycled by the body or excreted in our urine.
Aquatic animals like fish and amphibians however do not possess a biological method for converting ammonia, and thus eliminate it by direct excretion. This is mostly done through the gills for fish, and through the skin for amphibians. As ammonia is constantly being created in their bodies through a process called amino acid deamination, you can safely assume that your lovable little fishies are swimming ammonia factories. In nature the majority of this is diluted by the sheer amount of water they swim in; in captivity however, the volume of water most fish are kept in prohibits this from happening.
How does ammonia harm our fish?
Ammonia will directly harm your fish by burning the sensitive tissues of their gills. Just as exposure to gaseous ammonia will harm our lungs (at 35ppm atmospheric concentration, it can cause irreversible health problems in as little as 15 minutes of exposure), exposure to ammonia in aquarium water can harm your fish's gills. This has the direct and immediate result of hindering respiration, but also the long term inhibition of the gills to perform an acid-base balance (keeping the pH of the fish's blood at the proper level for biological functions to perform properly).
Ammonia exposure can also cause physical damage to internal organs, skin, and fins, immune system suppression, cause stunted growth, nervous system damage, and a whole host of other issues. Suffice to say, ammonia is never something you want in your aquarium in ANY
Signs of ammonia poisoning
Nitrite ( NO2−)
- Sudden death of stock
- Twitching, whirling, or other abnormal swimming
- An excess of body mucus
- Bloody patches on the body and fins
The second stage in the nitrogen cycle, Nitrite is no less dangerous than ammonia, but for different reasons. An ester of nitrous acid, nitrite has a nasty little habit of bonding with red blood cells and preventing them from moving oxygen throughout the body. This causes a condition known as hypoxemia, and can cause organ failure, nervous system damage, and brain damage. As a fish's gills essentially put their blood right out in the open, you can see how this can negatively effect their well-being in a short amount of time.
Imagine taking a breath, and every breath after that you grow progressively more and more tired. No matter how much you take in oxygen, you're always out of breath, until you die from organ failure. Not fun.
Symptoms of nitrite poisoning
- Heavy, labored, or otherwise impaired breathing
- Gasping at the surface or staying near oxygen-rich areas of the aquarium (air stones, intake tubes, etc.)
- Brown gill tissues
Nitrate is the least harmful of the three, but it can still have fatal consequences. 95% of the aquariums I test have at least a little bit of nitrates in them, and it's usually an unavoidable problem in most non-planted freshwater
aquariums. While most people consider nitrate harmless except in larger quantities, the truth is that even in concentrations higher than 30ppm nitrate can cause an inhibition of growth and development, cause nervous system damage and degradation (through the inhibition of vitamin B12 uptake, a crucial part of proper nervous system function), and cause general overall stress for aquatic animals. Nitrate has secondary ramifications in that it can be used as a food for algae, which in epidemic proportions can cause major issues with water quality and chemistry. Algal blooms can also lead to water anoxia (a lack of oxygen in the water).
Unlike ammonia and nitrite, nitrate is not converted in most home freshwater
aquarium systems into another substance. The most reliable (and easiest) way to lower nitrate levels in your aquarium is to do a simple water change. Chemical filtration methods are available to strip nitrates from the water, but these don't revitalize the ion balance of the water like a water change will.
Symptoms of nitrate poisoning
Nitrate poisoning is a little sneakier than ammonia or nitrite poisoning, as outward symptoms may not present themselves until the latter stages of the poisoning process. General listlessness, lack of feeding reflex, erratic swimming and behaviors, and overall poor health are all signs of nitrate poisoning; generally if these are noticed, your fish has suffered at least a little permanent damage. Your only recourse is to quickly rectify the nitrate issue and hope for the best.
Here's the good news: unless you are cycling a new tank, you won't generally see the first two unless something drastic has gone wrong with your aquarium. Having a good set of test kits on hand capable of testing these three compounds (in addition to pH, hardness, and alkalinity) is never a bad idea, and a water test should always be your first course of action when presented with an unknown problem. Having accurate numbers to present to us here also helps us help you. Learn the nitrogen cycle, learn how to properly maintain your tank, and with any luck The Big Three should never rear their ugly heads past the cycle process.