Nitrite poisoning in fish and new tank syndrome: how to save your fish

Nitrite poisoning happens when nitrite builds up in the water, usually in a new tank that hasn't finished cycling, and it stops your fish's blood from carrying oxygen. Do a large water change now, add a small, measured dose of aquarium salt (the chloride blocks nitrite from entering the gills), and keep testing daily until nitrite reads zero.

"New tank syndrome" is the name for the ammonia and nitrite spikes that kill fish in tanks set up only days or weeks ago. It is one of the most common reasons beginners lose fish, and it is preventable.

How to identify it

Your test shows nitrite above zero. This is the only sure sign. Any nitrite reading is a warning; ideal is 0.
Fish gasping at the surface or crowding near the filter outflow, even though the water is well aerated.
Fast gill movement, lethargy, fish resting on the bottom.
Brown gills. Healthy gills are bright red. Nitrite turns the blood chocolate brown, which is why it's called brown blood disease.
Fish dying a few at a time over days, often 2–5 weeks after setting up a new tank.

Where it fits in the cycle

In a new tank, fish waste makes ammonia. One group of bacteria grows and turns ammonia into nitrite. A second group, which usually takes longer to grow, turns nitrite into the much safer nitrate. So in a new tank you typically see ammonia rise and fall first, then nitrite rise, then nitrite fall to zero and nitrate start to appear. The nitrite stage is often the longest. See cycling.

Nitrite can also spike in an established tank after you clean the filter in tap water, overdose a medicine that kills bacteria, add many fish at once, or have a power cut.

When it's a problem

Early: nitrite of about 0.25 ppm, fish acting normally. Change water today and test daily.

Serious: 0.5–1 ppm or more, or any reading with fish breathing fast.

Emergency: fish gasping, lying on the bottom or dying, or nitrite at the top of the test chart. Act now.

Sensitivity varies a lot between species. Some fish cope with levels that kill others, and fish are more affected in warm water and when oxygen is low.

What to do right now

Change 50% of the water with dechlorinated water matched to the tank temperature. If nitrite is very high, do another 50% change a few hours later.
Add aeration. An airstone or extra surface splash helps fish whose blood is carrying less oxygen.
Add salt at a measured dose (see below). This is the most effective protection.
Stop feeding for 1–2 days, then feed very lightly. Less food means less ammonia and less nitrite.
Dose a conditioner that says it detoxifies nitrite (and ammonia), as a short-term bridge between water changes.
Don't clean the filter or replace the media. You need those bacteria.

What to do next

The salt dose

Chloride and nitrite are taken up through the gills by the same route. If there is much more chloride in the water than nitrite, the gills take up chloride instead, and the fish are protected. Fish farms keep chloride at least 10–20 times the nitrite level for this reason.

For a home aquarium, a low dose is enough: about 0.3 g of plain salt per litre (roughly one level teaspoon per 19 litres / 5 US gallons). Use aquarium salt, or plain non-iodised salt with no anti-caking agents. Dissolve it in tank water first, then add it slowly. This dose protects against the nitrite levels you'll usually see in a cycling tank, and most freshwater fish, plants, shrimp and snails tolerate it.

Salt doesn't evaporate or get used up. Only water changes remove it. Add salt only to replace what you take out, in proportion to the new water.
Don't keep raising the dose. More salt isn't more protection once you're well above the nitrite level.
Some catfish, loaches and soft-water fish are more salt-sensitive. At this low dose problems are rare, but watch them.
Once nitrite has read zero for a week, stop adding salt and let normal water changes dilute it.

Daily routine until it's over

Test ammonia and nitrite every day.
Change water whenever nitrite reaches about 0.5 ppm or ammonia reaches about 0.5 ppm. In a heavily stocked new tank this may mean daily.
Feed sparingly, every other day if needed.
Don't add any new fish.
Expect the nitrite stage to last 1–3 weeks. Bottled nitrifying bacteria or filter media from an established tank can shorten it.

Reading your test

Most hobby kits read nitrite as NO2. Some scientific kits read nitrite-nitrogen (NO2-N); multiply that by about 3.3 to get NO2. Know which yours is before comparing numbers.

Recovery

Fish usually recover quickly once nitrite is low again, as the body converts the brown blood back to normal. But nitrite also stresses the gills and immune system, so watch for ich, fin rot and fungus over the next two weeks. If you need to medicate, remove carbon and take care with scaleless fish, plecos, loaches, shrimp and snails, and with medicines that harm filter bacteria.

Products that help

Liquid test kit (ammonia, nitrite, nitrate): essential during the first two months of any tank.
Aquarium salt: protects fish from nitrite at a low, measured dose.
Water conditioner that binds ammonia and nitrite: a temporary bridge between water changes; it doesn't remove nitrite, and some test kits still read it.
Bottled nitrifying bacteria: can help a new filter establish faster; results vary between products.
Air pump and airstone: helps fish while their blood carries less oxygen.
Mature filter media from a healthy tank: the most reliable way to seed a new filter.

How to prevent it

Cycle a new tank before adding fish (a fishless cycle), or start with only a few hardy fish and test daily.
Add fish a few at a time, a week or two apart.
Never clean filter media in tap water; use old tank water.
Don't replace all the filter media at once.
Don't overfeed, especially in the first months.

Why it happened

The bacteria that remove nitrite weren't keeping up with the waste. That's normal in a new tank, and can happen in an old one after filter damage. Nitrite passes through the gills into the blood, where it changes haemoglobin into a form that can't carry oxygen. The fish suffocates even in well-aerated water.

When to get help

Get help if nitrite stays high for more than three or four weeks, if fish keep dying despite daily changes, or if your tap water itself tests positive for nitrite or ammonia. A local fish shop can often give you mature filter media to seed your filter.

In the game

Multiple Tank Syndrome doesn't have a nitrite poisoning card yet.