Nitrites

Nitrite (NO₂⁻) is an intermediate step in the nitrogen cycle, formed when beneficial bacteria break down ammonia. Although it's less toxic than ammonia, nitrite still poses a serious risk to fish—especially in freshwater systems—because of how it interacts with their biology. Understanding how nitrite affects your aquarium, and how to manage it, is crucial for keeping your fish healthy during the early stages of the cycle or if something throws your tank off balance.

Why Nitrite Is Dangerous

The main reason nitrite is so harmful has to do with how fish absorb ions. Nitrite is chemically similar in size and charge to chloride (Cl⁻), a vital electrolyte that fish actively transport across their gills. The issue is that nitrite can "sneak in" through the same transport pathways. Once in the bloodstream, it binds to hemoglobin more tightly than oxygen can, effectively disabling red blood cells from doing their job. This condition—called methemoglobinemia—is sometimes nicknamed “brown blood disease,” and it's not unlike carbon monoxide poisoning in humans.

Fish suffering from nitrite poisoning often exhibit symptoms like gasping at the surface, not because there’s a lack of oxygen in the water, but because their blood can’t carry that oxygen to the cells that need it. 

Salt as a Temporary Shield

One reason saltwater fish are generally less sensitive to nitrite is because of the high concentration of chloride ions already in their environment. These chloride ions compete with nitrite at the gills, reducing its uptake and protecting the fish. This same principle is why adding a bit of aquarium salt to a freshwater system under nitrite stress can help—it increases the chloride concentration and offers some degree of protection. Still, it’s a bandage, not a cure.

The real fix is dilution. Large water changes are the best immediate response to high nitrite levels, especially if fish are already showing signs of stress. After that, all you can do is give your biological filter time to catch up.

What Nitrite Tells You About Your Cycle

If nitrite is showing up in your tests, that means your nitrogen cycle is in progress. That’s good news—it means your system is moving in the right direction—but it’s also a sign that your tank isn’t yet fully stable. If you're cycling with fish, try to keep nitrite levels below 1 ppm to avoid stress. In a fully cycled, mature aquarium, nitrite should always read zero. Even low levels can suppress a fish’s immune system, making them more vulnerable to disease.

Ultimately, stable water chemistry is one of the best defenses you can give your fish. When nitrite is under control, their immune systems are free to do what they do best: keep your fish healthy.