A “cycled” fish tank refers to an aquarium that has beneficial bacteria in it that converts very harmful waste ammonia into less harmless nitrate. The cycle from fish to nitrate is called The Nitrogen Cycle.
Ammonia (NH3)
Ammonia is added to your fish tank as waste from your fish and the decay of protein (uneaten food or dead plants and animals). Ammonia needs to be kept under .5 ppm (parts per million).
Nitrite (NO2)
Eventually, if your aquarium has ammonia and gravel, an ammonia-oxidizing bacteria (AOB) that turns ammonia into nitrite will breed and loosely adhere to your gravel. Nitrite needs to be kept under .5 ppm.
Nitrate (NO3)
Eventually, if your aquarium has nitrite, a nitrite-oxidizing bacteria (NOB) that turn nitrite into nitrate will breed and loosely adhere to your gravel. Nitrate is safer for fish, so your nitrate only has to be kept under 40 ppm. Live plants can consume some of this waste nitrate, but it is best not to rely on that. Instead, our weekly partial water changes keep the nitrate in check.
Partial Water Changes
When you’re doing your partial water changes with your fish in the tank, you are keeping the ammonia and nitrite at safe levels for your fish. After four to eight weeks, your ammonia and nitrite are being kept at safe levels by being consumed by the bacteria, and your water changes are now only to keep the nitrate at a safe level.
Bacteria
You can often kickstart the speed at which the beneficial AOB and NOB grow by adding some to your fish tank. This is done by adding some gravel from one of your established tanks to your new tank, or by purchasing an aquarium bacteria starter product such as Seachem Stability or API Quick Start. If you get a bottle of the product with active bacteria still in it, it can significantly decrease the time for your nitrogen cycle to form.
To keep your bacteria alive, be gentle when cleaning your gravel, and don’t add chlorine (chlorinated water) or antibiotics to your fish tank. One sign that your bacteria have died is suddenly white cloudy water, but your bacteria could be dead even if you don’t see this.
New Tank Syndrome
New Tank Syndrome is when ammonia gets high in a new fish tank because it doesn’t have the bacteria for an established nitrogen cycle, killing fish. You can avoid this by starting your tank with a single fish, not overfeeding it, and doing weekly partial water changes.
Fishless Cycling: Establishing a nitrogen cycle in your tank before adding fish
You can also prevent this by establishing the bacteria (nitrogen cycle) before you add the fish in what’s often referred to as a “fishless cycle”. This is done by adding ammonia to the tank (with gravel and heater) at a level of 2 ppm. This is typically done by adding ammonium chloride.
Monitor the ammonia daily and add more if it drops below 2 ppm. Do not do any partial water changes during this time. Once you measure nitrites in the tank, and then later find the nitrites to be zero and nitrates above zero, you have the bacteria that you need. At this time, you just need to wait for the ammonia to drop near zero and do a partial water change if you need to keep the nitrate under 40 ppm, and then you can introduce the fish.
You will need a nitrogen test kit like the API Master Test Kit to follow these instructions. You should still do weekly water changes to keep your nitrates in check, but you may be able to go two weeks after you have established a nitrogen cycle.