Aquarium Cycling

How to Cycle an Aquarium.

Cycling a fish tank is the most important part of setting up a new aquarium and one that is not always explained to new fish keepers.  If large numbers of fish are added to an uncycled aquarium they will likely quickly die due to ammonia poisoning.

Why is Cycling Necessary?

As part of their natural lives, all fish excrete ammonia but unfortunately ammonia is very toxic to fish and ammonia poisoning is one of the most common causes of fish death.  In the wild the fish live in large expanses of water so the concentrations of ammonia doesn’t build up to great levels but in the aquarium, the volume of water is relatively low, so ammonia can quickly build up to dangerous levels.

The Nitrogen Cycle

In the aquarium, as in nature, the ammonia is taken care of by bacteria.  In the Nitrogen Cycle, ammonia is converted to Nitrite by nitrifying bacteria.  This Nitrite is then converted into Nitrate by a different type of nitrifying bacteria at which point it stays in the aquarium unless removed by a water change or taken up by plants as fertilizer.

Nitrite and Ammonia

Nitrite is also very toxic to aquarium fish as it interferes with the ability of haemoglobin to take up oxygen.  Thankfully Nitrate is less toxic but at high levels it can still be problematic for fish, which is one of the main reasons why it is necessary to undertake regular water changes.

Fish-In Cycle

In order for the cycle to start, the aquarium needs a source of Ammonia.  This is why it is not possible to fill a fish tank with water and just wait for it to cycle.  Traditionally, aquariums have been cycled using a hardy fish such as a Zebra Danio or a Platy.  The theory is that the fish will generate the ammonia needed to start the cycle and the relatively small amount of ammonia that is produced should not kill the fish whilst the bacteria numbers build up.  Once the bacteria have built up to a level that they can nitrify the quantities of ammonia produced by one fish, more fish can be added very gradually.  The advantage of this method is that the fish tank can be started immediately but the obvious disadvantage is that the fish are exposed to ammonia and potentially nitrite poisoning.

To minimise the amount of ammonia the fish are exposed to at one time, the fish should be fed sparingly and water changes should be performed on a regular basis.

Fishless Cycle

These days the fishless cycle method is becoming more and more popular.  In this method, a small amount of chemical ammonia is added (about 2ppm), which is available from various stores (I got mine from Boots) and then the bacterial are left to tackle the ammonia without the presence of any fish.  The aquarist should test for ammonia every day using a readily available test kit until the ammonia level reaches 0.25/0.5ppm, at which point more is added to bring the total up to 2ppm.

At this point, the aquarist should also be testing for Nitrite, which is produced from the nitrification of ammonia.  It is likely that nitrite will be quite high at first as it takes time for the correct bacteria to be established but when both ammonia and nitrite is reduced to zero in one day, the filter is ready for fish.  If fish are not purchased straight away, the filter bacteria need to be kept fed by regular additions of ammonia.  Obviously when the fish are added, no more ammonia is added as the fish will generate the food source for the bacteria.

It is also a good idea to check pH levels as well as ammonia and nitrite throughout the cycle as the water will likely to become more acidic during the process and filter bacteria do not work very well in acidic environments (although ammonia is much less toxic to fish at low pH levels).  If pH levels appear to be crashing then adding bicarbonate of soda will help raise it – add this slowly and test after each addition to make sure the correct amount is present.

The advantage of a fishless cycle is clearly that no fish are subjected to toxic ammonia and nitrite but an added advantage is that the aquarium can be stocked with much more fish as a group of small fish will not produce more than 2ppm of ammonia which the filter can already deal with.  The disadvantage of the fishless cycle is that it can take a long time to complete so a lot of patience is required

Speeding up an Aquarium Cycle

There are ways in which to speed up a cycle.  Probably the most fool-proof way is to seed the filter with bacteria from a mature filter from another aquarium.  This can be done from other fish tanks the aquarist may have, from a friend’s aquarium, by outreaching on social media or even from a local fish shop.  Alternatively there are products that can be purchased that can seed an aquarium filter, although there is quite some variation over the effectiveness of each product.

Hopefully the above will give a good outline of how to perform both fishless and fish-in cycles, which are an essential part of setting up an aquarium.