Learning the tank cycle can be a long and confusing process, but bear with me! I would consider understanding the tank cycle to be the most essential part of fishkeeping, and you certainly won't regret learning about it. An uncycled tank can be dangerous and even lethal for your fish, as ammonia will quickly harm and kill any fish living in even small concentrations.

The Cycle is what happens in your filter. Filters can be Internal, External (Canister), Hang on Back (HOB) or even an extra tank called a "Sump". Within the filter you have what's called filter media. This media comes in lots of different forms - some common forms are:

  • Ceramic noodles
  • Plastic "bio" balls
  • Filter wool
  • Filter sponge/foam (coarse and fine)
  • Pumice, scoria and glass "rocks"
  • Activated carbon (sometimes in the form of filter sponge)

The bolded media is commonly used as what's called "Biological media" (because of the large amount of "clingable" surface area). The unbolded media is commonly used as what's called "Mechanical media" (allows for good flow, catches particles and waste). The biological media is what we'll be discussing in this article.


Within your biological media is lots of gunk (what we refer to sometimes as "marmite", and you can see why after a month of running a filter!) which contains good bacteria that will work on your cycle. There are two types of bacteria in this - the bacteria that turns ammonia into nitrite, and then the bacteria that turns nitrite into nitrate.

I'll get to the technical bits in a second, but this next bit is very important: do not go cleaning your filter weekly! This was the first thing I and many others did when we started fishkeeping. A clean filter is a clean tank, right? Surprisingly, nope! Because your good bacteria live on the biological media, it needs to stay there. Sometimes you need to clean your filter to stop the "gunk" from building up - when you do this, it's best to only clean the sponge/foam or 25% of the "mechanical" media. This means you clean out any built-up waste without harming too much of the good bacteria. If you clean out the biological media, you risk killing all the bacteria on it which causes the tank to go into a "mini cycle". When rinsing the mechanical media, it's important to do so in the old tank water from a water change, so that no cold/chlorinated water kills any remaining bacteria.


So wait, what was all that gibberish?

When your fish create waste, they excrete what's called ammonia. Ammonia (NH3, or Ammonium, NH4+) is incredibly toxic to fish, and needs to be turned into something else. Your first set of bacteria do just this, and turn the Ammonia into Nitrite. Nitrite (NO2-) is the second part, and is also quite toxic! The second set of bacteria quickly get onto this, and turn that Nitrite into Nitrate. Nitrate (NO3-) is the final part of your cycle. Nitrates are then able to be removed through both water changes and plants, although you need a very heavily planted tank to remove equal or more nitrates than what are being produced, and even then water changes are necessary to re-add minerals in the water which are important for the plants and fish.


So, how do I get these bacteria?

The process of cycling is how you get these bacteria. You can cycle in three ways: Fishless cycling, Fish-in cycling, and Pre-seeded bacteria.

Fish-in cycling can be considered cruel, as even the hardiest fish can be heavily affected by the ammonia produced (as, remember, there aren't any bacteria yet to convert it) and ammonia burning can have long-term effects on the fish such as "ammonia burns" to the gills. For this reason, I don't encourage fish-in cycling. However, for some it can be too late, and for that, don't panic! It just means you'll have to do a bit more work than usual. Over the next few weeks, your filter will cycle itself through the waste produced by the fish. This form of cycling simply means you need to do daily water changes to make sure the ammonia stays to an "acceptable" level. Water changes will help dilute the ammonia and reduce the stress on the fish - however, this means that the cycle may take longer than usual, and it is important you don't add any new fish until the cycle is finished (and once it is, add them in very small amounts to not shock the system).

Fishless cycling is my preferred method of cycling. This method is super easy, and requires very little maintenance. In order to create and feed the bacteria, you need to be producing waste. There's a couple of ways you can do this:

  • You can "feed" the tank ammonia on a daily basis. Pure ammonia can be bought from your local supermarket - just make sure it doesn't contain any extra nasties! There isn't a real guide on how much to feed the tank, but it doesn't matter too much as there's nothing currently living in it.
  • Literally feed it! On a daily basis, feed whatever you would be feeding your fish to the tank and let it rot.
  • My chosen method: Putting a prawn in the tank. Put a prawn in a filter sock or some pantyhose (just to keep it in one place) and let it rot in the tank for the next few weeks. This means you can leave the tank running without any maintenance, although I like to do a 100% water change (remember your dechlorinator!) afterwards to clean up any mess.

Seeding the filter is possibly the easiest way to go, but you need to have some handy. Simply take about 25% of the media (and replace it with new media) from an already established filter, and put it into your new filter. I like to run the dechlorinated water through the filter for about a day before adding the seeded media, just to make sure it isn't shocked by the new water. After this, you can add fish instantly! Be careful as to only add a few fish (or inverts) at the beginning, as your bacteria need to spread to the rest of the filter material, but it's also important to add fish as soon as possible or to continue to feed it until you add fish as the bacteria will quickly die without anything to feed on!

Almost all of my tanks came from the media of my first ever tank, and I have never had any issues with pre-seeded filters, however as you would with the other methods it's important to test your water for the first week to make sure it's safe for the fish!


How do I tell if I've finished cycling?

When you first start the cycle you'll expect to find high ammonia. Then you'll find some ammonia, and some nitrites. You may then find some ammonia, nitrites and nitrates - then nitrites and nitrates, and then simply nitrates. Once you find nitrates, your cycle is finished!Here's a chart of what your cycling tank may look like:



Once cycled, your tank should then test these levels - if they're not like the ones below, it means your tank is still cycling. If you have an established tank which is showing the levels below, it could mean that your tank is either overstocked, your filter is not strong enough or your tank is going through a "mini cycle" (where for some reason the bacteria has died and needs to cycle again) -

Ammonia: 0ppm

Nitrites: 0ppm

Nitrates: 5-20ppm

Even the heavily planted tank rarely has 0 nitrates, and no nitrates actually means you may have tested wrong (don't worry, the nitrate test can be confusing!). Most fish will be OK with nitrates up to 60ppm, but nitrates above 20ppm can stunt growth and stress a fish out, which means that diseases might thrive on your fish. Sensitive fish such as Otocinclus sp. can even die in nitrates above 40ppm. Some other fish will handle nitrates up to 80ppm! All fish are different, but as a general rule nitrates shouldn't go above 20ppm. If in doubt, do a water change, as will help lower your nitrate levels and give your fish fresh, clean water.

Note that some tap water can have nitrates up to 40ppm, but most sits around 5-10ppm. Having nitrates of 40ppm simply means that it is advised to not keep very sensitive fish, unless you are interested in running the water through a clean planted system to help lower the nitrates before it goes into your tank.


What do all these colours mean, and how do I test these levels?

The colours on the chart above are based on API's Test Kits which are available from most pet stores. The liquid test kits are by far the most accurate way of testing your water, as the "paper dip" tests or "stick on" tests are often unreliable and could lead to the death of your fish if they fail. These tests are simple and easy to do, and are good to do on a weekly basis (and should especially be the first test done if a fish ever shows sign of illness).


A note, however, is to take extra care when doing these tests. If dosed wrong, they may give false readings, leading you to think your levels are either safe when they are not, or too dangerous! A difficult test to do is the nitrate test, and is the one I most often seen done wrong. Read and follow all the instructions carefully - it is very important that the second bottle is shaken for the time instructed, and that the test tube is shaken and left to sit for 5 minutes before getting your results. I've even seen someone who was getting 0 nitrates, when they had nitrates over 120ppm! But don't worry if you get it wrong the first time - it was a while before I realized they were giving me false readings, too.

These test kits are a bit pricey, but in the end they are every fishkeepers best friend. If you cannot afford one now, most pet stores are happy to test your water for free. If you have one right now, go test your water! You may be surprised as to what your readings are.


In conclusion, the cycle is an extremely important part of fishkeeping and it is important you understand it. If anything is confusing, don't panic, as it is often confusing to everyone when they first read it. Don't be afraid to ask questions, and feel free to message us, or ask your local fishkeeping forum!