The Nitrogen Cycle
The Cycle:
A newly setup aquarium or pond does not contain very many bacteria. Once fish are introduced, ammonia (NH3) will begin to be added to the tank from uneaten food and fish wastes. In acidic waters (pH < 7), the ammonia will combine with hydrogen cations (H+) to create ammonium (NH4+) which is not very toxic. If the levels of ammonia become too high, they will burn the fishes' gills and make it hard for them to breathe. The fish will zip all around and die. Luckily, there is a naturally occurring bacteria in natural waters that converts ammonia into nitrite (NO2-). Over a few days or weeks, these bacteria will begin to grow in a new aquarium or pond. Then, the levels of ammonia go down while those of nitrite go up. Unfortunately, nitrite is even more poisonous to fish and kills them more easily. Again, luckily, there is a second bacteria that converts nitrite to nitrate (NO3-). This takes a few more weeks. Nitrate is only harmful in very high concentrations. Live plants use ammonia and nitrate as food. Water changes reduce the concentrations of ammonia, nitrite, and nitrate.
Changing Water During the Cycle:
Much has been said about the nitrogen cycle and its importance. For people who change very little of their water and not often, this is very important. Also, for ponds, it is important initially. But, if you have just a few fish, add bacteria, and change 30-50% of the water every week with treated similar water, then the nitrogen cycle is not so important (ammonia and nitrite will never be readable). Some aquarists say not to change water as the cycle gets set up but I believe it is better for the cycle to take a bit longer with water changes than to kill or almost kill the fish in a rush to get the cycle "completed." Changing water during the cycle will only delay its establishment, it will not halt the process.
Bacteria:
Beneficial Bacteria which has live bacteria in it to help the nitrogen cycle. There are many such products (Cycle is another) and conflicting reports on whether they do any good at all. In water above pH 7, two groups or species of bacteria (controversy now abounds on what species actually do it) convert ammonia to nitrite to nitrate (NH3 =>NO2- => NO3-). In acidic water, it is ammonium (NH4+) which starts the process, which is less toxic than ammonia. Nitrite and ammonia can kill fish easily. Nitrate is less toxic and provides food for many live plants and algae. The helpful bacteria take a month or two to get up to full speed with the fish waste load. A pond may take a year to grow a healthy colony of bacteria. The bacteria do best between 70 and 90 degrees F. They die in too cold or hot water. In cold water, fish usually hibernate and produce less waste; therefore, the bacteria are not needed in winter. The key to keeping fish safe from their own wastes and other sources of ammonia and nitrite is to make water changes with vacuuming and to add only a few fish at a time when setting up a tank
Cycling a New Tank and Fish Choice:
A tank can be cycled using fish (actually their wastes, other aquatic animals work as well) or ammonia added directly. Cycling a tank with fish means adding fish so that they produce ammonia to promote the growth of the bacteria that convert it to nitrite and then nitrate. A tank is "cycled" when ammonia and nitrite readings are sustainably zero (they stay zero for at least a few weeks), and nitrate readings increase. Many people tell new aquarists to cycle their tanks with cheap or trash fish like feeder fish, danios, guppies, etc., and then to get rid of them or return them to the store. I do not believe this to be a good choice. First, these fish are not respected and being treated poorly are most likely full of diseases, bacteria, and/or parasites. These are then introduced into the new tank and stay there. Second, after the tank is cycled, the fish have to go somewhere. If unwanted fish were used, they either go back to the store (in which case the stress most likely kills them) or are killed outright. It is much better to use fish that one intends to keep to cycle the tank. There is NO reason that a single initially-healthy fish should die during cycling (see information above on water changes). Anyone who tells you that it is "normal" for a fish to die is misinformed or trying to sell you more fish.
The fish desired in the tank should be determined ahead of time. A few of the most hardy of those should be the first inhabitants of the tank that will get the nitrogen cycle going. These fish will remain in the tank when the tank is cycled, and new fish are added. Good cycling fish that can be kept in most community tanks include fancy guppies, small danios, small tetras, small barbs, white cloud mountain minnows, etc. Even most so-called sensitive fish can be used to cycle a tank without dying. In rare cases, it may actually be necessary to cycle a tank with fish that cannot remain in the tank forever (like a piranha tank). Ideally, these fish would be moved to a tank of their own after cycling. Algae eating and bottom dwelling fish such as plecostomus, otocinclus, algae eaters, snails, corydoras catfish (cories), and freshwater shrimp should not be added until after the tank has been set up for a while and algae and bottom debris are present.
What to Do if the Ammonia or Nitrite Spikes too High:
Often, in small tanks (10 gallons or less) or overstocked tanks, the nitrogen cycle can be deadly. This occurs when the ammonia (and later nitrite) produced by the fish, food, etc. is simply so high that it begins to harm and kill fish. Once bacteria become established, the levels will go down to zero but what to do in the mean time? First, if the levels of ammonia or nitrite ever read in the test kit's danger zones, doing nothing will most likely kill the fish. The most important thing to do initially is to get the levels down to acceptable ranges. For tanks under 50 gallons, doing a 50% water change (less water in larger tanks) with gravel vacuuming EVERY day until the levels are below the danger zone (but still present) will help ease the stress on the fish and yet allow the good bacteria to develop. Yes, it will take longer but then the fish might actually survive. The addition of liquid bacteria like Stress-Zyme or Cycle or dry bacteria in larger ponds especially, will help get the good bacteria growing. Because ammonia (NH3) is in the form of ammonium (NH4+) when the pH is acidic, lowering the pH if the aquarium water is basic (above 7) will render the ammonia less toxic to the fish. The pH may be lowered using commercial solutions or with vinegar (acetic acid) but should be done slowly and carefully. Liquid Ammo-Lock by Aquarium Pharmaceuticals can be added to bind some ammonia if present. Zeolite (a white granular compound, that looks like white rocks, often sold mixed with carbon) can be added to the filter (in a mesh bag or pantyhose or media container) or even put right in the tank or pond (in a bag of some sort) to absorb ammonia as well. Adding large doses of salt at least a tablespoon per 5 gallons will help detoxify nitrite if that is the problem. Keep in mind that salt is harmful to plants and some sensitive fish in high doses. Once nitrate test kits register nitrate present, the danger has most likely passed. This takes about a month or so.
Also, note that salt should be used sparingly if zeolite is also being used. This is because zeolite can be refreshed using sodium salts which replace the ammonia in the zeolite. If lots of salt is added to a tank or pond with zeolite that is full of ammonia, the zeolite may release noticeable amounts of ammonia which could be harmful. Use the zeolite to remove ammonia and later, if nitrite becomes too high, use salt to render it less toxic and remove the zeolite. Using small amounts of salt in a tank or pond with zeolite should not make much difference.
Some ways to reduce the cycling time.
- Use filter media from a cycled tank
- Use 50% of cycled tank water for new tank
- Use of bottled bacteria products
Some water conditioners like Seachem Prime helps too if u intend to add the fish in slightly earlier :D