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How to Cycle Your New Fish Tank With Fish

Why Do Fish Die in a New Tank?
When you set up a new fish tank, establishing the biological filter, known as 'cycling' the aquarium, is the most important thing you can do to ensure the fish you put into the tank don’t die within days. Failure to do it will almost certainly result in the 'new tank syndrome,' which is just as scary as it sounds. You lovingly set up the aquarium, fill it with crystal clear water, carefully select your fish, slowly place them in the tank making sure that you give them time to acclimate, and everything looks great for a couple of days. But soon all your new pets are gulping frantically at the surface, then going belly up and dying.
The dead fish fell victim to ammonia a by-product of their metabolism. All fish produce ammonia and it is lethal to them. In lakes, ponds and rivers, the volume of water is so big, that the ammonia produced by fish is diluted to undetectable levels. However, in a home aquarium, the volume of water is much more limited, and ammonia levels quickly rise and kill the fish. Although the water in a new tank appears very clean, it is in fact toxic.

What Is 'Cycling' a New Fish Tank?
Cycling refers to establishing populations of special bacteria in your aquarium and filter which will carry out the nitrogen cycle. These nitrogen fixing bacteria actually like ammonia, they will take it from the water and turn it into relatively harmless nitrate. This is in fact a two step process, first one type of bacteria produce nitrite, which is still harmful to fish, then a second type will take the nitrite and metabolize it to nitrate.
You don’t actually need to add the nitrogen bacteria to your aquarium, they are everywhere and will naturally seed the biological filtration sponge in the filter. The problem is that when you first set up your new fish tank, the bacteria are present in very small numbers. There is not enough of them to cope with the waste of your fish. As soon as ammonia levels begin rising, the ammonia fixing bacteria start to multiply, but it takes time for them to fully populate the filter sponge. After a certain period of time, ammonia levels start falling, however, nitrite levels will start rising, this stimulates the second type of nitrogen fixing bacteria to multiply and start using up the nitrite. After a while nitrite levels will start falling. A properly cycled tank has undetectable levels of ammonia and nitrite.

Cycling With Fish:

Set up your aquarium and filtration system.
To start, you'll want your aquarium completely assembled and filled with everything you want in it, besides the fish. Below is a brief checklist of things you'll want to do before getting started — this may not match all aquariums perfectly:
Assemble the aquarium
Add substrate
Add water
Add air stones, air pumps, etc.
Add plants, rocks, etc.
Add filtration system
Add heater

Introduce a small number of hardy fish to the tank.
Your goal in this cycling process is to populate the tank with fish that produce waste but can survive the initial high levels of toxins long enough for the beneficial waste-processing bacteria to grow. Thus, you'll want to pick a variety that is known for being a good cycling fish and start with a small number. Later, once the bacteria have grown, you can slowly add more fish of different types. Below are just a few good choices for cycling fish:
White Clouds
Zebra Danios
Cherry or Tiger Barbs
Pseudotrophius Zebra
Banded Gouramis
X-ray Tetras
Pupfish
Most minnows
Most guppies

Feed fish sparingly.
When cycling an aquarium with your fish, it's very important not to overfeed them. Though different fish may have different dietary needs, a good rule of thumb is to give food once every other day. Only offer a moderate-sized meal — you don't want any extra food left over when the fish are done eating. This is done for two reasons:
Fish who eat more produce more waste, which can cause the levels of toxins in the tank to rise before the bacteria have a chance to colonize the aquarium.
Leftover food will eventually rot, producing toxins on its own.

Perform frequent water changes.
While you're waiting for your tank to cycle, every few days, replace about 10-25% of the tank's water. As with the reduced feeding schedule described above, this is another way of ensuring that toxin levels don't get too high before the bacteria has a chance to grow. If you have a saltwater tank, don't forget to add a suitable amount of marine salt every time you change the water to keep the tank at a proper salinity.
Don't use chlorinated water — this can kill the bacteria in the tank, forcing the cycle to start over. If using tap water, make sure to treat it with an appropriate dechlorinator or water conditioner before adding it to your aquarium. If using bottled water, make sure to buy distilled water, as "purified" or "drinking" water may have minerals added for taste that may be harmful to fish.
Be ready to perform water changes much more frequently if you start to see signs of serious ammonia stress in your fish. However, try to avoid stressing the fish by exposing them to big changes in water chemistry or temperature.

Use test kits to monitor toxin levels.
When you add fish to your tank, the levels of toxic chemicals known as ammonia and nitrites will quickly rise as the fish release waste into the water. As beneficial bacteria begin to grow in response to these chemicals, their levels will gradually drop to near-zero, at which point it's safe to add more fish. To monitor these chemicals, you can use commercially-available test kits, which are usually sold at the same places fish and aquariums are. Testing daily is ideal, but you can sometimes get away with testing every few days.
You will want to keep ammonia levels below 0.5 mg/L and nitrite below 1 mg/L throughout the cycling process (ideally, they should be less than half of these values.) If these chemicals start to approach unsafe levels, increase the frequency of your water changes.
The cycling process is complete when both ammonia and nitrite levels drop so low that they are undetectable. For practical purposes, this is often referred to as "zero," though this isn't technically accurate.
As an alternative, you can take water samples to the pet store where you bought your fish or aquarium. Most will offer cheap testing services (some even do it for free!)

Add additional fish gradually once toxin levels are near-zero.
The cycling process typically takes about six to eight weeks. Once ammonia and nitrite levels are so low that they aren't showing up on your tests, you may add more fish. However, you'll want to do this gradually, introducing just one or two new new fish at once. Adding just a few fish at a time keeps the increased amount of ammonia and nitrites in the tank from each new addition well within the ability of the bacteria to control.
After each addition of new fish, wait at least a week or so, then test the water once more. If ammonia and nitrite levels are still low, you may add your next few fish.

Next Week: Performing "Fishless" Cycling