Cycling the Aquarium for Beginners

So you decided to get some fish, Goldfish are the easiest ones to start with Yes? Unfortunately, no!

You have a tank, and you realised that you need to dechlorinate the water. Happy days!

Not really…

Unfortunately, goldfish are not the easiest fish to begin with – they are particularly messy, and harder to care for than you may have imagined… But stick with it and you will get there!

All fish and fish food creates a waste product called Ammonia in your tank, which can be deadly to your fish – remember that most fish evolved to live in massive volumes of water, not a small tank. What size is your tank? Is it big enough? Most Fish Shops want to sell fish, and will not necessarily advise you that the tank you are proudly buying to really look after your fish is WAY TOO SMALL, unfortunately.

I was that person….

The ideal, best and only recommendable way to get fish is to NOT GET FISH, initially, and do a fishless cycle, but most people arrive on help sites such as this only after they realise their first and perhaps biggest mistake. It is not unfixable, but you are going to have to put a lot of effort into keeping those inexpensive little fish alive for the next few weeks. If you care about their lives, and / or your child’s happiness, follow on…


In a properly cycled aquarium, Ammonia is converted to Nitrites, which is in turn converted to Nitrates, by special bacteria which will grow in the filter of your tank. Many people only realise about this important cycle when they arrive at a forum such as this with sick or dying fish.

Ammonia is deadly, Nitrates slightly less so, and Nitrates are not good, but least deadly of all.

It is therefore obvious that IDEALLY we would all cycle our tanks before buying a fish – how?



Bacteria need time to replicate – yes we all know that they do grow rapidly, but time is the only factor in a fishless cycle.

Firstly, chlorine kills bacteria, so you need to have dechlorinated water in the fish tank – I had no idea at all about that as a child, but I also read that it will burn their gills, causing pain! But for a fishless cycle, you are protecting only the bacteria, as there are no fish!

A fishless cycle is the best way to get the tank ready for your new friends – you will have a strong filter, which will be able to deal with everything they can throw at it!

Given the size of the tank, about which you can read about elsewhere, on this forum, your filter should be able to turn over the water approximately 10 times per hour – so a 200 litre tank needs a 2000litre per hour filter. In the USA a 60 gallon tank needs a 600gph filter.

  1. Let’s get started. Fill the tank with dechlorinated water, add any substrate (gravel or sand you like), but most people here would recommend a bare bottom tank, as it is easier to keep clean, and there is no risk of the fish eating a piece of gravel (and “choking”). Some bugs can live on the tank wall and in the substrate, but most will live in the oxygen rich media of the filter.


  • Plants could be added at this stage – they love ammonia, but I would feel that they might compete with what you want to win – the bugs. I would go bare tank and no plants, personally, at least to begin with…


2. For this step you need to find a source of pure ammonia. The brand you might use depends on where you live – In the US, it seems to be Walmart Janitorial Ammonia which is widely recommended. In UK and Ireland, Kleeneze pure ammonia. But as long as it has only ammonia and no additives or detergents/fragrances any brand SHOULD be OK.


  • Important! Read the label! Plenty people say that if you shake the bottle and it froths, it is no good (detergents!)


The ammonia eating bacteria need a source of ammonia to survive, which is in your bottle – there will be very few in your tank until you “entice” them in with some decent food. Apparently they can swish around in the air or in your tap water, so eventually a few will arrive in your tank…. Is it worthwhile leaving off the tank lid – I don’t know, but as there are no fish to leap out…

None of what I am about to advise is anything that was not information gained by reading the (Almost) complete guide to fishless cycling, which is an excellent article. It is well worth a read…


3. Add the Ammonia……. Using your API Freshmaster test kit to check as you go, you need to raise the Ammonia to a level of about 4 parts per million. You could try to measure the ammonia by volume – a 200 litre tank = 200000 ml,  so one ml of pure ammonia would give you a concentration of 0.000005 – 5 parts per million. You will need to read the bottle – is the ammonia pure, or is it diluted with water?


4. Once the concentration is about correct – better to go under than over, you need to wait and be patient – eventually the ammonia levels will begin to fall, and Nitrites will begin to show. Even later, nitrates will begin to be produced. All you need to do is to be patient and keep monitoring ammonia levels every few days – it is important to know when the ammonia level has begun to drop, as your bacteria need a food source, and you don’t want to starve them!


5. Keep adding ammonia to the tank to keep the original concentration. Extra oxygen and heat will help the bacteria to grow, so if you have an airstone, put it on, and a heater if you have one.


6. You should eventually arrive at a situation where you add ammonia one day, and it has all been consumed by the next day. So test for nitrites – the bacteria which eat nitrite will come along later – their food source is not there until the first part of the cycle is working, so there will probably be a Nitrite spike – as the Ammonia is being converted to Nitrite.

  • Important: Excessively high levels of ammonia or nitrite can actually slow down the cycle process. If your test reads above 4 ppm ammonia or above 2 ppm nitrite, do large water changes until you can bring the levels back down into the optimal range. 

Keep testing for both, keeping the ammonia levels up, until the nitrites also begin to fall. When you test zero for both ammonia and nitrites, congratulations!


7. Next – add ammonia to one part per million, and check that 24 hours later those two toxins have returned to zero. If it has your tank is now cycled, but NOT YET ready for fish! The concentration of Nitrates in the tank will be MASSIVE by now – at a dangerous level for the fish, so you will need to do a massive water change.


  • All or most of the bacteria are in the filter; I would change 100% of the water in the tank. You will now need to use a few drops of ammonia per day, or some fish flakes to produce a smaller amount of ammonia in the tank to keep the bugs alive, until there are fish to do that for you, but most people will rush to get fish as soon as they can.


Important: Nitrates will be reduced slightly by plants in the tank, if you have any, but once you have added fish you will anyway need to do regular partial water changes to keep on top of the nitrates, and to keep the water fresh – there are minerals and stuff in the water which can best be replaced with fresh water.


Fish-in Cycling – A FISH IN CYCLE is what you will need to do if you already have fish in your tank – it is the same principle, but it is more difficult, because 4ppm of Ammonia would kill fish in minutes!

Important: You will now need to keep the ammonia and nitrite much much lower in the tank, to keep the fish safe, which can mean that a fish in cycle will tank longer – there is less ammonia. It may take up to 6-8 weeks, or even longer.

Fish will produce ammonia so you will not need to add pure ammonia to feed the bacteria. I would keep testing ammonia daily, and if it was to reach 0.5ppm on the API test, do a 50% water change and keep testing daily. Eventually ammonia will begin to reduce, so now you will need to also test for Nitrites (should not exceed 0.5 ppm), though I would probably recommend testing for both from the outset. Again if either reaches a dangerous level, do another 50% water change.


Some folks here swear by certain products to make a fish in cycle easier, but I personally had little luck with them. Where I live the only one I found was Nutrafin Cycle, which claims to add the beneficial bacteria to the water. API make a similar product ?? Safe start.

And you can also get products which can detoxify the ammonia into a safer form called ammonium. Many here recommend Seachem Prime, a product I have never seen in Ireland. This should keep the fish safer, but I was always concerned that the API will still detect the “good” Ammonium, as well as the bad, meaning that it still looked dangerous on the test. So I continued to do the large daily water changes, to keep the levels low. Perhaps I was being overly cautious on that point?

Another way to get good bugs is from a friend – if you know a fish keeper, perhaps ask them for a bit of floss from their filter – it will contain bacteria which can be transferred to your tank, but their tank MUST be healthy, or you may import things you wish you had not!


A fish-in cycle is by no means the end of the world, but for the next few weeks the tank is going to require very regular testing – I would suggest daily, and regular large partial water changes -if you Ammonia is reading 0.5, a 50% water change will reduce it to 0.25. Lots of fresh water will not harm the fish, as long as you match the temperature, and remember to dechlorinate the water!

Good Luck