The Nitrogen Cycle
Much has been written about the natural process that occurs in the fish tank to convert harmful fish waste (ammonia) to more benign waste (nitrate), so we will skim the subject here.
Ammonia hinders the uptake of oxygen in the guppy’s blood, causing stress. High concentrations cause stress. Gill filaments become irritated and begin to swell. Oxygen supply is cut off to the membrane cells. They become infected with bacteria. Kidneys are damaged or become infected. Ammonia can affect growth, and reproduction.
Different strains of guppies have different levels of tolerance to ammonia poisoning. One of our strains is an “indicator.” If one or two guppies from the strain are found lying on the bottom of the aquarium after a water change, we know that the ammonia levels spiked.
About 60 percent of the ammonia present in a tank is excreted by guppies through their gills. The balance comes from bacterial breakdown of fish feces and urine, uneaten food, and other organic matter you may have introduced to the tank. Nitrogenous waste products break down into ammonia (NH3), which is highly toxic to fish, especially at high pH values. In fish tanks it can take just a few hours for ammonia to reach toxic levels when the nitrogen cycle is not functioning.
The nitrogen cycle is the “big fish eat little fish” story in reverse.
The fish eat food, what is not used by the fish becomes nitrogenous waste and breaks down to ammonia. Ammonia (NH3) is consumed by nitrifying bacteria (Nitrosomonas spp).. What they don’t use they excrete as nitrite (N02-). Another species of bacteria (Nitrobacter spp.) consumes nitrite and excretes nitrate (NO3-). There the process stops. However, nitrate as the end product of the cycle can build up over time and become toxic in high enough concentrations. This makes water changes a necessary part of maintenance.
How much is too much ammonia in your tank? It depends on the pH level of your tank and the temperature. The following chart gives the maximum long-term level of ammonia-N in mg/L (ppm).
|pH||25C (77F)||20C (68F)|
If the ammonia test kit you buy at the pet store can detect the presence of ammonia, you probably have too much in your tank and the nitrogen cycle is not functioning properly.
How much nitrate can fish tolerate? Concentrations in excess of 400 mg/L appear safe for fish, according to laboratory studies.
Cycling a New Tank
When you set up a new tank, and put tap water into it, it is virtually sterile. This is called “new tank syndrome.” Without the presence of the bacterial colonies that convert fish waste to relatively harmless nitrate, fish can become sick. This graphic shows how the nitrogen cycle develops over time in a newly established tank.
Illustration by Philip Shaddock
Along the bottom of the graph is the time it takes to cycle a tank (three to four weeks). Along the side are concentrations of ammonia, nitrite and nitrate in parts per million (ppm). The chart shows an average cycling time. Tanks can cycle faster or slower than this depending on a number of factors. Test the water with a test kit. Once the level of nitrites falls, and nitrates can be detected, you can start introducing guppies into the tank.
There are three methods for “cycling” a new tank.
One is “fishless cycling” where a product like ammonium chloride is used to initiate nitrifying activity. Ordinary household ammonia is used, providing it does not have additives or perfumes. Four or five drops of ammonia per 10 gallons are added each day until a nitrite spike occurs. Then the ammonia is cut back to 2-3 drops per 10 gallons per day until the nitrites levels drop to the point where they cannot be detected. Guppies are not placed in the aquarium until bacterial colonies become established. People who use this method claim that a colony of nitrifying bacteria can be established in as little as two weeks and it is big enough to support the introduction of a large amount of guppies.
Gradual Cycling with Culls
The second method is the use of culls to initiate the cycle. In a sterile tank, release a couple of your healthy but unwanted guppies and let them take the brunt of the cycle. Feed them very lightly and do 10% water changes often (every day if you can). Because the cycle is not fully functioning throughout the cycling period, it’s very important that you feed them very lightly. You do not want the culls to get sick, which makes this process somewhat risky. When you do move other guppies into the tank, move them in gradually, giving the tank time to support the added bioload.
The third method is to fill the new tank with water from established tanks, and move a filter from a seasoned tank into it. There is a higher risk of disease in this method. However we have used this method successfully for years without mishap. The only precaution we take is to run the tank a couple of days before moving fish into it.
If we introduce new fish to the fish room, we place them in a newly set-up tank, instantly cycled. However, we add Melafix and Formalin to knock back the existing diseases in the tank, to give the new inhabitants time to develop their immunity to them. Sometimes we will use potassium permanganate to lower the level of organic particulates in the water from other tanks. See the article on potassium permanganate.
Note that if water conditions in the tank are significantly different from the tank the filters came from, you may damage the bacteria colony in the filter. Always treat a bacterial colony with the care you would your guppies.
If the ammonia levels are detectable during cycling, and there are fish in the tank, use a product like Amquel or Ammo-Lock to neutralize the ammonia. It will still be converted to nitrite and thence to nitrate. After dosing the tank, do a 25% water change. In the following days, do daily 10% water changes.
Products available from stores that claim to jump-start the cycle process are of doubtful value. Nitrifying bacteria require the presence of oxygen, and these products usually come in sealed containers. There has been no scientific confirmation of their claims to speed up cycling.