February 15, 2023

5 Steps to Tropical Aquarium Success Part 4

Adding Fish and The Nitrogen Cycle

Buying and Adding Fish

Our staff at Wharf Aquatics can help you with selecting appropriate fish for your aquarium, but you may find it useful to do a little research in advance, so that you have an idea of the range of fish available, and the type of fish you would like.

Observe fish carefully before buying. Make sure all fish in the tank look healthy, and avoid any with damaged fins or gills.

When adding fish to the tank, allow the bag to float in the tank for about 15-20 minutes to equalise temperature.

Add only a few fish initially to a new tank - otherwise they will suffer serious effects of New Tank Syndrome due to Cycling (see below).

Feed lightly - this will help to minimise pollution during the unstable period as the tank matures. Overfeeding is one of the most common mistakes made by beginners to the hobby. Add food in small amounts and watch while the fish eat it.

Consider setting up a quarantine tank for new stock.

The Basics of Cycling

The term "cycling" is applied to the process which takes place as a new aquarium matures. This refers to the build up of essential bacteria, which break down wastes

The implications of this are probably the single most important factor in maintaining healthy fish.

Nitrogen Cycle in an Aquarium

In a new aquarium, there are not enough of the bacteria to cope with the waste load, and toxic ammonia can rise to dangerous levels. Eventually, the bacteria increase to cope with the ammonia, converting it to another, only slightly less toxic compound - Nitrite. This too will then rise to high levels until a second type of bacteria increases and converts it to the much less toxic Nitrate.

This process can take several weeks. In the confines of an aquarium, there is not really a complete "cycle", and most of the end product, Nitrate, is normally removed by water changes as shown in the diagram above.

There are therefore certain things which should be done to minimise any stress or even fatalities during the cycling period:

  • Add only a few hardy fish at first and feed lightly to minimise wastes.
  • Test water regularly and perform water changes to reduce the levels of ammonia and nitrite if they become dangerously high. The bacteria are attached to surfaces, so removing water should not slow down the maturing process.
  • If possible, obtain some gravel, tank decor, plants or filter media from a mature tank. This will introduce some of the necessary bacteria and may reduce or even eliminate cycling time.

The aquarium is considered "mature" when ammonia and nitrite have reduced to zero, and nitrates have begun to rise. At this point it will be necessary to begin a regular program of water changes to keep the level of nitrates low (aim for less than 50 mg/l, less than 25 mg/l is better).

Fishless Cycling

This is a method which uses ammonia to start the nitrogen cycle in a new aquarium, rather than fish wastes. Essentially, the ammonia solution (pure household ammonia with no additives, or a product like Biomature) is added to the aquarium daily. When the ammonia-converting bacteria have established, the ammonia reading should fall back to zero overnight. The addition of ammonia is continued at a reduced level to keep the process going as nitrites rise and then subsequently fall as the nitrite-converting bacteria become established. When nitrites read zero, the tank is cycled and a water change is carried out to reduce any resulting nitrate before fish are added.

The advantages of this method are obvious - no fish are exposed to the toxic waste products during the cycling process. It also means the aquarium can be stocked with more fish straight away, as there is a full compliment of bacteria established. The fish wastes essentially take the place of the ammonia additions.

It is also possible to carry out this fishless cycling process by adding fish food to an empty tank for a few weeks and monitoring waste levels as above, although this method tends to take longer.

However, it should also be noted that a fishless cycle is not a perfect replacement for the natural maturing process in an aquarium - for example using an ammonia source doesn't build up the bacteria which break down more complex wastes, and the process also requires careful monitoring with test kits.

Introduction Equipment Tank Decor Adding Fish Maintenance