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The Art of Fishless Cycling

Fishless Cycling an Aquarium

Cycling the Aquarium

What is it? This is building a colony of nitrifying bacteria in your aquarium without the traditional use of ‘disposable’ fish providing the ammonia food that the bacteria need to grow. Why do this? Placing fish in a tank of freshwater will soon kill them within a week or less. Fish constantly pee toxic ammonia into the water and this chemical is deadly if not removed somehow or diluted like it is in a river, pond or lake. So we need to find a way to remove it from our aquariums. As luck would have it, there is a number of naturally occurring bacteria species (which is harmless to us) found on virtually every surface on the planet which can eat this ammonia and turns it into a much less toxic chemical which is easily removed at our leisure. How do we collect this bacteria and make use of it?

Surprisingly easy. When you buy an aquarium it usually comes with a motorised filter which has a sponge which is inside it, the filter sucks water in and passes the ammonia ladened water through the sponge. As this bacteria (called nitrifying bacteria) will be naturally on the sponge (but in very, very low numbers) we must encourage it to grow in size on the sponge, eating the ammonia the fish give off until the bacteria colony is of sufficient size of numbers to keep our fish fit and healthy. The process of building your bacteria colony is called Cycling and when the colony is of the right size to remove all the toxins, we call it ‘Cycled’. The problem is that it takes 4–8 weeks for the bacteria to grow on the sponge to sufficient numbers to consume all the ammonia the fish gives off. During this time the fish can die or become seriously ill for the rest of its life. In the past, aquarists have used certain species of fish which can withstand a small level of ammonia in the water without dying. These hardy fish excrete their waste products into the water and so feed the bacteria colony on the sponge with their own ammonia. However in recent years this has being frond upon because of extra expense of buying these fish, the possible pain or death and assured long term health problems these fish will go through as a result of being exposed to the toxic by-products of their own waste. So people came up with alternative ways to feed the bacteria colony to get them up to size without harming any fish at all. The method I’m going to talk about in this article is called ‘fishless cycling’. We simply add raw ammonia from a bottle into a tank with no fish in it. We monitor the toxic levels until we are satisfied that the bacteria colony is eating them all. The tank is then ready to accept fish!

The trick is doing this fishless cycle as fast and efficiently as possible. There are a lot of articles online about this process but most are flawed. They take little or no account of the scientific requirements of the bacteria in order to grow it as fast as possible. I’ve spoken to various companies and scientists in order to find out the bacteria’s ideal environment. Benefits There are several benefits now being highlighted by aquarists by using this method. 1. Less expense. You don’t have to go out to a shop and buy fish, which you may not even want to keep, once the aquarium is fully cycled. 2. The moral aspect. Every year more studies seem to show that fish feel pain. Perhaps in not the same way as mammals, but certainly they react to it and will display avoidance behaviour. Aquarists often claim certain species of fish are more hardy against being exposed to their own toxic waste. However the science doesn’t bear this out. Fish in the wild are not exposed to ammonia or its cousins nitrite or nitrate in the levels that we subject them to in tanks due to the huge volume of ponds, rivers water, so they’ve not evolved to be tolerant. They have never had the need. Also the vast majority of fish on sale in fish shops are youngsters. They are at a critical growth stage in their life. If they are exposed to these toxins, they naturally enough will not develop a healthy immune system, organs or bones. You will probably stunt their growth and make them unwell. It’s well known in the aquarium hobby that goldfish placed in unfiltered tanks are the most abused fish by beginners and this is probably why so many remain small or die within a month. Indeed, some people still believe that goldfish will only grow to the size of their tanks and remarkably few people have seen large goldfish in the home. 3. More control. By monitoring the levels of ammonia yourself and adding it only when required you achieve a faster cycle. Research what the bacteria need If you’re planning to grow bacteria on a sponge then it’s reasonable to have a grasp of what they need to grow well and provide these conditions. Often this stage is overlooked and people wonder why the cycle time is taking too long a time. Expectations A full cycle takes at least 25 days if you’re lucky. You can’t speed this time up (the bacteria will only grow so fast as day) without taking a shortcut or two. Shortcuts There are way to speed up the growth of the bacteria and so reduce the time you spend on it. Add more bacteria This can be done in a number of ways: Add some of the water from an established tank.

This isn’t so good a shortcut. Bacteria don’t live in large numbers suspended in water. They prefer attaching themselves to surfaces. Plus there is the danger of exposing your new tank to unwanted possibly parasitic life living in the water to your new tank.

Add some gravel from an established tank.

The surface area of your typical gravel pebble is low and will therefore not harbour much nitrifying bacteria and if you add blackened gravel from under the substrate surface, you’re actually adding anaerobic bacteria which paradoxical may actually produce toxins or hold lifeforms you don’t want. Then there is the hassle of once you’ve removed the gravel from the aquarium you have to keep it wet and you have an hour or so to get it back into fresh oxygenated water as the bacteria will quickly use up the oxygen dissolved in the transport bag and die!

Add some brown muck from an established filter.

This is an excellent way to boosting your levels of bacteria. The brown muck is billions of bacteria clumped together! Again you’ll need to get this muck into your tank quickly – within an hour. But like the previous short cuts, you do run a risk of introducing snails, food scraps and other creatures or diseases from the existing tank to your new tank. Make sure you can trust the source.

Add some soil to your tank.

A bag of garden soil from your local garden shop doesn’t cost much and by placing a thin layer of this soil in your new tank and then covering it with a 1″ layer of gravel, you create what is called a Walstad or ‘El Natural’ tank. The billions upon billions of bacteria in the soil provides you will tens of thousands of helpful species which can not only perform the nitrifying process, but also breaks down fish waste so saving you maintenance work and provides a more natural and healthier environment for fish. See the Walstad article for more details on this super fast method of cycling a tank.

Add some bacteria from a commercial bottle.

This is the most expensive method. But ultimately the fastest method. Until ~2003AD there were a lot of dodgy bacteria products selling nitrifying bacteria which were the wrong species for the aquarium and therefore did little to speed up the cycle time. But thankfully these are now few and far between. If you choose a modern one that claims to greatly speed up or virtually eliminate the cycle time then it probably does. You can realistically cycle a tank within 2 to 5 days using this products. See Bacteria bottles, do they work for more detail on this interesting subject.

But in this article I want to talk about performing the basic fishless cycle, using no added bacteria from any source and only a bottle of ammonia. It is a cheap if lengthy method. But it works.

Bacteria and their needs The nitrifying bacteria we need to nurture have specific needs to grow well. If you don’t know what these are, then you’re relying on luck to get you through. The Species We are primary concerned with two groups of species of bacteria – Nitrosococcus (Nitrosococcus mobilis related)- This gram-negative bacteria (2 microns in dia.) converts ammonia into nitrite. Nitrospira (Nitrospira briensis related) – This bacteria is the slower growing of the two and converts nitrite into nitrate. (For a long time the hobby was told it was Nitrosomonas and Nitrobacter species. But modern research proved this to be incorrect in aquariums.) Their needs Temperature.

These two species need water that has an optimal temperature of ~28°C (82.4°F) . The bacteria growth rate is determined by the temperature. The warmer it is, the faster they grow. But there are limited to this. Studies have shown that above 30°C (86°F) you stress and kill off these bacteria. So don’t over heat.


The bacteria ideally need a pH of around 7.6-8.0 to grow. They just don’t want to grow around a pH of 6-6.9. If you want an aquarium with acidic water, wait until the cycle is complete and then alter the pH to your desired value over the course of a few days to allow the new pH to take hold and not kill off the bacteria due to a single swift change.


This is the level of Calcium or Magnesium dissolved in the water. The bacteria needs calcium and other trace minerals to build their cells. With a low GH they can’t grow. Aim for a GH of at least 6dH. Calcium carbonate (often called ‘garden lime’ in garden shops or Coral sand in aquarium shops) is used to raise GH or Calcium bicarbonate (a bread additive) can be added if GH and KH is low. So easy to find. If you’ve got hard tap water then this isn’t a problem as high GH levels make water hard.


You’ll need a decent level of KH in order to buffer the pH against shifts towards the acid that will occur as the nitrifying cycle starts. Your bacteria will eat KH as it grows. If you live with hard tap water (high GH), then this level shouldn’t be a problem. But with soft water areas you’ll need to add Sodium bicarbonate (Bicarbonate of soda) to raise KH and keep it above ~6dH. Bacteria, like all life, don’t like a wildly fluctuating pH due to low KH.


These bacteria need a lot of oxygen to grow. Every 1 mg of ammonia that is converted to nitrate takes 7–8 mg/L of oxygen and there is precious little enough oxygen in ammonia soaked water. So ensure a good flow of well oxygenated water into the filter media. This is why wet/dry filters do so well building their colony, they have constant exposure to the air. Internal filters by their very design have their inlet positioned at the bottom of their box and therefore get water from the least oxygenated part of the aquarium. You need to keep oxygen levels above 4 mg/L. Ideally by having the outlet of internal filters pouring the return of water over the water surface so it breaks the surface and stops a possible bio-film forming on the surface stopping the water getting oxygenated. If concerned, add addition aeration via a airstone and air-pump to break up the surface of the water. Tip: position your motorised internal box filter upside down (or horizontal) so the inlet is just under the water surface where the most oxygenated water is. This ensures the bacteria inside gets lots of well oxygenated water! Then when the tank is cycled, righten it.


Studies show that bacteria is built up of many trace elements including calcium and carbon. But luckily ordinary tap water has an abundance of these. Keeping GH above 6dH will ensure the bacteria get the vital calcium they’ll need.


Nitrifying bacteria is sensitive to UV light. So no direct sunlight falling on the tank. Built-in tank lights have minimum UV light and so are not a problem. But unless you’ve got a tank with plants, don’t put the aquarium lights on unless you have to. This will hinder algae feasting on the ammonia instead of your bacteria!

Getting Started You will need at the very least: A container to hold water.

You can cycle a tank from any size. But it’s always easier to do it with a bigger body of water over 40 Litres (10.6 US G.). The temperature of the water is easier to control and the chemistry of the water is more stable. But a minimum practical body is probably 15-20 Litres (4-5.3 US G.) litres as the hobby doesn’t make filters for tanks smaller than this.

A filter of some kind.

This is a device which draws aquarium water into itself continuously and flows it over and through the media (usually plastic foam or sponge in small filters, or glass or ceramic beads or balls in large filters). The bacteria will live and grow on the media. This must be left switched on 24/7 and don’t change it! Internal sponges will last for years and don’t need changing. Even if your filter has a black activated carbon section (and the manufacturer says to change it periodically) don’t change it for the first two months as you’ll seriously delay the cycle time!

A thermostat or heater.

It is essential that the water is kept within a certain temperature range. Whilst there is no fish in the tank, you may think that the water temperature doesn’t matter. But the bacteria have a ideal temperature to grow rapidly. If you can’t control the minimum temperature then the speed of the cycle may be lengthened as nitrifying bacteria prefer it warm.

A thermometer.

You’ll need to measure the temperature of the water. Electronic ones are nicer if you can afford it (~£10) and some will give you a history of the minimum/maximum water temperature which can be useful if the room every gets too hot or cold.


I know some people prefer a bare bottomed tank for ease of cleaning. But the surface of glass in a tank isn’t a great media for nitrifying bacteria to grow on, though you will get some. So without the benefit of extra surfaces like gravel to grow on, be aware that your cycle length may lengthen if you omit this.

A Chlorine/Chloramine tap water conditioner.

Tap water contains chemicals designed to render water safe for us to drink. However these are toxic to nitrifying bacteria and your fish. Your tap water supplier may use Chlorine or Chloramine. So any water you add to your tank needs to be pre-treated with what we call a ‘water conditioner’. This bottle renders these toxic chemicals harmless to the bacteria. I recommend always buying a water conditioner which treats Chloramine as it treats Chlorine as well so you’ll not have to worry if your tap water supplier decides to using Chloramine and they didn’t tell you! If you don’t know which type of chemical you have coming out of your tap. Ask your supplier!

A bucket.

Handy things buckets. Buy one and only use it for aquarium use. Don’t put any soap, bleach, detergents, household items or old rags in it as you may contaminate it! Remember soap and other detergents kills bacteria and fish, so don’t use them anywhere near your aquarium!

Rinse out your tank with raw room temperature tap water to remove dust, etc. and lastly fill up the tank with water and leave for 15 minutes and then remove the water. That should remove/kill any unwanted bacteria or creatures that were on the inside surface of the tank. Be careful not to scratch the inside of your tank if it is made of plastic, it’s easily done! Please note, this isn’t a comprehensive guide on how to set up your tank, seasoned aquarists know new tanks need examining for leaks and that should take place after 24 hours. Take your gravel, put it in a bucket with water that has been water conditioned to remove Chlorine/Chloramine (you don’t want to kill the bacteria that is naturally on it) and rinse it so the water runs clear to the eye. Add this gravel to the tank to a depth of about 2.5cm (1″). The more time you take over this the better. Add filter, heater, water that is at room temperature and switch on equipment. You don’t add cold water to a room temperature aquarium, the shock could crack it! Let the water settle for 30 minutes (you can add ornaments during this time) and measure the temperature, pH, GH and KH to ensure they are within the optimal levels discussed above. If you’re water is soft, then add small quantities of bicarbonate of soda to get the pH close to 7.6-7.9, with the GH and KH above 6dH. Once these levels are stable for at least 30 minutes, you can add the ammonia source.

The ammonia source This is where methods of fishless cycling differ.

Meat, prawns and other organic food Some people use decaying food to rot away providing ammonia. But this is very messy as the food breaks up into tiny pieces and becomes uncontrollable as you have no easy way to limit the ammonia levels being produced. Then of course the reason it is decaying in the first place is that you are promoting unknown species of bacteria and fungi to feast on it. Decaying food adds phosphate to the water and along with the ammonia these chemicals will help feed unwanted algae growth. The decay process will also removes oxygen from the water which would otherwise be available for the nitrifying bacteria. Ammonia from a bottle I strongly recommend using ammonia liquid from a bottle. This is relatively easy to get hold of. Here in the UK I found it was the smaller DIY shops that were selling it. It is a powerful cleaning liquid and some shops will often hide it away as it is so toxic if children get hold of it! So you may need to ask for it. Look for a brand that list its ingredients and that they don’t include any soaps, detergents or additives like perfume. These may be fatal to the bacteria. A good brand in the UK is Kleen Off.

Dosage Next you need to find out the dosage you need. I found my brand of ammonia was making my 15L tank test for 1ppm of total ammonia with 5 drops or 0.5ml. I’d add a few drops and wait two minutes before testing the ammonia level again. So on the first day add enough ammonia so when you test, you get 1ppm.

I’ve seen other articles that add much more. But this is asking for trouble. Studies have shown that the ammonia to nitrite bacteria is fast growing and more tolerant of environmental conditions. But the nitrite to nitrate bacteria is slower and more sensitive to toxins. In cases of doing a fishless cycle it is often reported that the nitrite levels keep increasing without any nitrate showing at all. This is why – The nitrite to nitrate eating bacteria is itself intolerant of high levels of nitrite and ammonia. So if you add lots of ammonia, you’re actually inhibiting the bacteria growth. So keep ammonia levels at 1ppm to give it a good start. By the second or third day you may find that the ammonia level has decreased, this is to be expected and isn’t due to any remarkable super-fast bacteria growing. It’s due to some of the ammonia turning into gas due to the amount of water turbulence you’ve set up. So top it up. As the days pass Simply keep topping up the ammonia level as required so you’ve got a consistent 1ppm level in the tank. Don’t forget that over the days to come, add water conditioned water as it evaporates and then add the ammonia. Do daily checks on the flow through the filter and check water temperature is advisable. By the end of the first week you can expect to see scum or bio-film developing around the water edges but it shouldn’t be on the water in any quantity, you’ll need to break up the surface more otherwise. You want maximum oxygen in the water. Don’t expect to see any nitrite in the first week.

As the cycle progresses, the bacteria will make the water slightly more acidic as the weeks pass so keep an eye on pH and KH. You shouldn’t be having any serious algae growth as this plant needs light to grow. If you think you are getting some, then turn off the lights. You don’t need them unless you’ve added plants in that case reduce the lighting by decreasing their on duration. As everyone’s start up is unique with different amounts of bacteria lying around, all I can do is say keep the level of ammonia constant and wait for the level of nitrite to start to climb. After a couple of days with nitrite in the tank, you should see nitrate start to register on your test kit hopefully. If not, and nitrite is getting above 3ppm then you will need to dilute the water in order to keep the nitrite down below 2ppm and think about reducing the ammonia level to 0.5ppm temporary to give the Nitrospira (nitrite to nitrate) bacteria a helping hand to start growing. This bacteria as I’ve said before is sensitive to excessive toxins when in low numbers. But once nitrate is growing, increase the ammonia back to the 1ppm level and monitor it daily. When nitrite disappears you can be confident it’s all being converted to nitrate. When it’s cycled When you find that the 1ppm ammonia level is disappearing after 6 hours as well as you measuring no nitrite then you can say your tank is cycled. The last stage is to ensure you build up your two colonies of bacteria so it is sufficient to support the amount of fish you wish to add. Increase the dosage of ammonia from 1ppm to a 2ppm level and measure daily until ammonia and nitrite disappears within 6 hours of adding the ammonia. Then do it again with 3ppm and again with 4ppm. This way you’ll build up large and strong colonies. Just remember that you’ll need to keep adding ammonia every day until you get your fish/frogs/snails/shrimps/reptiles introduced. Your bacteria will die off within a few hours of having no ammonia to feed upon!

Written by –Quatermass 11:59, 25 April 2010 (UTC)

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