Nitrate Poisoning in Freshwater Aquarium Fish (+8 Prevention Tips)

Excessive nitrate in aquariums is frequently misinterpreted by aquarium enthusiasts and can lead to nitrate poisoning — which can kill aquarium fish. Nitrate is chemically identical to nitrite in that both are composed of oxygen and nitrogen molecules, however, nitrate is a less reactive substance.

Nitrite has just two nitrogen atoms for every oxygen atom —making it a more reactive molecule, whereas nitrate has three nitrogen atoms for every oxygen atom. Nitrate is a natural byproduct of fish waste and the final product of the nitrogen cycle, and it will accumulate in any tank that is not properly managed.

Nitrate Poisoning vs. Nitrate Shock

Nitrate poisoning is known by two different names depending on the severity of the exposure: nitrate poisoning refers to a chronic problem. In contrast, nitrate shock refers to an acute problem that happens suddenly.

What Is Nitrate Poisoning?

Nitrate poisoning occurs when fish are subjected to steadily growing nitrate levels over an extended period of time when normal tank maintenance is not carried out.

Overfeeding and overstocking of fish are also key contributors to nitrate levels slowly growing. But be aware of other sneaky causes of increased nitrate levels — like old tank syndrome.

When steps are not taken to reduce nitrate levels — like utilizing a regular maintenance schedule — the cumulative effect can be fish death. Some fish will be affected by levels as low as 20 mg/L while others will show no apparent symptoms until levels have reached several hundred mg/L.

Juvenile fish are affected at the lower levels, as are saltwater fish. For most aquariums, it is recommended to keep the nitrate level below 20-40 mg/L by performing regular water changes.

What Is Nitrate Shock?

Nitrate shock occurs when fish are suddenly exposed to a drastically different quantity of nitrate, which can range from several hundred mg/L to several thousand mg/L. When fish are transplanted from one aquarium with normal nitrate levels to another with high levels, this happens. Although nitrate shock generally refers to a fast spike in nitrate levels, fish can also be stunned if nitrate levels drop dramatically.

Symptoms and Mortality

New and sudden behaviors in your fish may indicate that they may be dealing with a nitrate problem in your tank over the past 24-48 hours:

Appetite loss
Rapid gill movement, rapid respiration
Dazed appearance, listlessness
Disorientation and loss of balance
Lying on the tank floor
Head-to-tail curling (advanced stages)

When nitrate levels gradually increase over time, it’s not uncommon for only one or two fish to develop symptoms at first. Until water tests are completed to check the water’s chemistry, there may be no obvious explanation for why certain fish become unwell. More fish will begin to display symptoms if nitrate levels are not decreased. Fish death will eventually happen — usually over a few days to a few weeks.

When fish are unexpectedly exposed to extremely high nitrate levels, they generally die within 24 hours. Owners are frequently unaware of the condition until the fish are dead or near death. In these cases, emergency remedies can be taken for affected fish, but the cause of the problem must be identified and removed to avoid more tragedy before adding new fish to the tank.

To combat this common issue, a continuous sensor that provides constant nitrate readings will help save your fish. The Seachem Ammonia Alert uses sensor technology that continuously detects and monitors free ammonia. Here’s the product in action:

Most fish will not perish suddenly if they’re gradually exposed to high nitrate levels. It’s feasible to minimize the effect of nitrate poisoning in circumstances of sudden exposure to excessive nitrate, providing the fish with a fighting chance of survival. Perform a water change and test the water for ammonia, nitrite, and nitrate levels. Ammonia and nitrite levels should be nil, while nitrate levels should be less than 10-20 mg/L.