The Nitrogen Cycle and New Tank Syndrome

Quick answer

The Nitrogen Cycle and New Tank Syndrome

Most fish that die in the first weeks of a new aquarium are not killed by disease. They are poisoned by their own waste in water that looks perfectly clear.

Fish excrete ammonia continuously, mostly straight out through the gills. In an established tank, colonies of bacteria living on the filter media convert that ammonia into nitrite, and then nitrite into far less harmful nitrate, which you remove with water changes. A brand new tank has none of those bacteria. Ammonia climbs, then nitrite climbs, and both are lethal long before the water looks dirty.

• Ammonia burns the gills. Nitrite destroys the blood's ability to carry oxygen. Both cause fish to gasp at the surface in water that has plenty of oxygen in it.
• The bacteria live on surfaces, above all inside the filter, not in the water. This is why swapping the water does not move the cycle, and why washing the filter under the tap can undo months of progress.
• A tank that has simply been running empty for a week is not cycled. Without an ammonia source, nothing grows.
• The safe method is to cycle the tank before any fish go in. It takes weeks and it is the single most valuable thing a new fishkeeper can be patient about.
• If fish are already in an uncycled tank, water changes are the treatment. Not medication, not a filter change, not waiting it out.

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If your fish are gasping at the surface, sitting on the bottom with clamped fins, or dying one after another in a tank set up within the last two months, test ammonia and nitrite immediately and do a large water change with dechlorinated, temperature-matched water.

This is an emergency and the clock runs in hours. Do not add medication, do not add more fish, do not strip and restart the tank, and do not wait to see whether things settle. Water changes physically remove the poison, and nothing else available to you does.
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Decide in 60 seconds

Why a new tank kills fish

Nothing in the tank is processing waste yet.

On day one, an aquarium is a glass box of water with a pump in it. The filter is not a filter in any biological sense; it is a sponge with no population living in it. Every gram of ammonia the fish produce stays in the water until something eats it.

Ammonia climbs first.

Within days of adding fish, ammonia becomes detectable. It damages gill tissue directly, causing swelling and thickening of the delicate membranes where gas exchange happens. The fish becomes progressively less able to extract oxygen, which is why it moves to the surface where oxygen is highest, and why it gasps.

Ammonia also damages the skin, the eyes and the nervous system. Fish that survive an ammonia spike often show reddened gills, cloudy eyes, ragged fins and a susceptibility to infection for weeks afterwards.

Then bacteria arrive and nitrite climbs.

Ammonia-oxidising organisms colonise the filter and start converting ammonia to nitrite. This feels like progress, and the ammonia reading falls, but nitrite is not an improvement. Nitrite crosses the gills and converts the fish's haemoglobin into methaemoglobin, which cannot carry oxygen. The blood turns brown, which is why this is sometimes called brown blood disease.

The cruelty of this stage is that the fish is suffocating in water that is fully oxygenated. Aeration alone does not fix it.

Finally nitrite-oxidising bacteria appear and produce nitrate.

These are the slowest to establish, which is why the nitrite phase of a cycle typically lasts longer than the ammonia phase. Once they are working, nitrite falls to zero and nitrate begins to accumulate. Nitrate is the tolerable end point, not because it is harmless but because it is far less acutely toxic and because water changes remove it.

When both read zero, the tank is cycled.

The test is not that the readings look low. It is that the tank can convert a full day's worth of ammonia to nitrate within twenty four hours, leaving ammonia and nitrite both at zero.

How to cycle a tank before you buy fish

This is called fishless cycling, and it is the method to use.

1. Set the tank up completely.

Substrate, decor, plants, filter running, heater running at the temperature the fish will eventually live at. Warmth speeds the bacteria up considerably, so a cold tank cycles slowly.

2. Dechlorinate the water.

Chlorine and chloramine kill the bacteria you are trying to grow, as well as harming fish. Treat every drop of tap water that goes into the tank, at every stage, forever.

3. Add an ammonia source.

The bacteria need food or they will not multiply. Options are a bottled ammonium chloride solution sold for the purpose, a pinch of fish food left to decay, or a small piece of raw prawn in a mesh bag. If you use a household ammonia product, it must be pure, with no detergents, perfumes or surfactants, and most household products fail that test.

Aim for a modest reading rather than a high one. Very high ammonia stalls the process rather than accelerating it.

4. Test daily and write the numbers down.

Use a liquid reagent test kit. Dip strips are convenient but are widely regarded as unreliable for ammonia and nitrite, and this is exactly the situation where a wrong reading has consequences.

You are watching for a sequence: ammonia rises then falls, nitrite rises then falls, nitrate appears and climbs.

5. Seed it if you can.

The fastest legitimate shortcut is mature filter media from an established, healthy tank: a squeezed sponge, a handful of ceramic media, some used substrate. Ask a trusted local fishkeeper or a good aquatic shop. Bottled bacteria products vary a great deal in what they actually contain and how they were stored, so treat them as a possible boost rather than a guarantee.

6. Confirm it before stocking.

Dose ammonia again and test twenty four hours later. If both ammonia and nitrite read zero the next day, the filter can handle a full day's load. If either shows, it is not finished.

7. Do a large water change, then stock slowly.

Nitrate will be high at the end of cycling, so change a lot of water before the fish arrive. Then add a few fish at a time and allow the bacterial population to grow into each increase. Stocking a fully cycled tank to capacity on day one overwhelms a filter colony that only ever grew large enough for the ammonia you were dosing.

Expect the whole process to take several weeks. Cooler water, low pH and a poor starting bacterial population all make it longer. There is no reliable way to make it take days.

Rescuing a tank that already has fish in it

If fish are in an uncycled tank, you are doing what fishkeepers call a fish-in cycle. It is not a technique to choose deliberately, but it is a situation many people find themselves in after buying a starter kit and a bag of fish on the same afternoon.

Test ammonia and nitrite every single day.

This is not optional. Without daily numbers you are guessing.

Change water whenever either reads above zero.

The size of the change should be enough to bring the reading down to zero or as close as you can get. Always dechlorinate, always match the temperature. A large well-matched change is far safer for the fish than leaving them in poison.

Water changes do not slow the cycle down in any way that matters. The bacteria are in the filter and there is always more ammonia coming.

Feed sparingly.

Less food means less ammonia. Small amounts every other day is enough for adult fish for a few weeks. Remove anything uneaten.

Add no more fish, and no medication.

Many medications, especially antibacterials, kill filter bacteria and make the situation worse. The fish look ill because of the water, and the water is what you treat.

Consider a dechlorinator that also detoxifies ammonia.

Some water conditioners bind ammonia into a form that is much less toxic to fish but still available to the filter bacteria. These are a genuine help during a fish-in cycle. They do not remove the need for water changes, and they can make certain test kits read confusingly, so keep testing and keep changing water.

Increase surface movement.

Damaged gills extract oxygen poorly. Extra aeration will not fix nitrite poisoning, but it gives a compromised fish every advantage.

Ask about chloride if nitrite is the problem.

Nitrite enters the fish through the gill pathway that normally absorbs chloride, so raising chloride in the water competes with that uptake and offers real protection. Aquarium salt is the usual source. Some fish, some invertebrates and many plants tolerate salt badly, so ask an aquatic vet or a knowledgeable aquatic retailer what is appropriate for your particular stock rather than adding it blind.

What a healthy, cycled tank looks like

Ammonia zero. Nitrite zero. Every test, every week, without exception once the tank is established.

Nitrate present but modest, and falling after each water change. A slowly climbing nitrate reading is the normal sign of a working system.

Fish spread through the tank at their normal levels rather than crowded at the surface. Fins held open. Normal colour. Eating with enthusiasm.

Gills pale pink and moving at a steady, unhurried rate.

Water clear, though clarity by itself proves nothing. Ammonia and nitrite are invisible and odourless in the tank.

Why an established tank suddenly crashes

The filter media was cleaned in tap water.

Chlorine kills the colony on contact. Rinse media in a bucket of old tank water instead, and only enough to clear the worst of the debris.

All the media was replaced at once.

Cartridge-based filters that tell you to throw the cartridge away monthly are throwing the bacteria away with it. Where a cartridge must be replaced, run the old and new side by side for several weeks first.

The filter stopped for a while.

A power cut, a pump that clogged, or a filter unplugged during maintenance and forgotten. These bacteria need oxygen, and a filter that has been still for a long period can contain a dead, decomposing bed. If a filter has been off for many hours, it is often safer to rinse the media in tank water and expect a partial recycle than to switch it straight back on.

Medication was added.

Antibacterial and some antiparasitic treatments hit the filter. Where medication is genuinely needed, a hospital tank protects the main system.

Too many fish were added at once.

The colony grows to match the load it has been given. Double the load overnight and it takes days to catch up, with an ammonia reading in the meantime.

Something died and was not found.

A fish behind the filter or a snail inside an ornament produces a large, sudden ammonia load.

Overfeeding.

The most common cause of all, and the easiest to fix.

The substrate was deep cleaned.

Stripping and scrubbing everything at once removes a substantial share of the biological surface area along with the dirt.

What never to do

Do not add fish to a tank to start the cycle. Using animals as ammonia sources causes real, prolonged injury.

Do not run a tank empty for a few days and call it cycled. Without an ammonia source nothing grows.

Do not wash filter media, sponges or ceramic rings under a running tap.

Do not replace all your filter media at the same time.

Do not do a full tank strip down and rebuild because the fish look ill. You will remove the biological filter at the moment the fish most need it.

Do not treat gasping with medication before testing the water.

Do not stop water changes because you have read that the cycle needs the waste to build up. The bacteria always have more than enough ammonia; the fish are the ones running out of time.

Do not rely on ammonia-removing resin pads as a permanent solution. They exhaust, and they mask the problem while starving the filter.

Do not raise pH or add buffers while ammonia is present.

Do not keep fish in an unfiltered bowl. There is no biological filtration in a bowl, so there is no cycle, and the water is toxic between every change.

When to get help

Contact an aquatic or exotics veterinarian, or an experienced aquatic service, the same day if fish are gasping at the surface after a water change has been done, if gills are red, swollen or bleeding, if fish are dying in succession, or if ammonia and nitrite will not come down despite repeated large water changes.

Seek advice within a few days for fish that survived a spike and then developed ragged fins, cloudy eyes, cotton-like patches or red streaking, since ammonia damage opens the door to infection.

Ask a knowledgeable aquatic retailer or vet before adding salt, medication or any pH adjuster to a tank that is cycling, since all three can make the situation considerably worse.

Bring your written test log with dates, your tap water test results, the tank volume, the filter type and when it was last cleaned, the full stocking list with dates each fish was added, your feeding routine, and the size and timing of every water change you have done. In this problem the record is the diagnosis, and a tank with no numbers behind it is very hard for anyone to help with.