Nitrite Poisoning

Symptoms

Spotting illness early gives the best chance of a cure, so check your fish daily at feeding time. Rapid or laboured breathing, gasping, lethargy, and brownish gills (the blood can no longer carry oxygen). It follows the ammonia stage of an uncycled tank. If you see several of these signs together, act promptly rather than waiting.

Causes

Nitrite builds up as the filter only partly cycles, binding to fish blood and starving it of oxygen ('brown blood disease'). In nearly every case the underlying trigger is stress, poor water quality, or the introduction of an infected fish - all of which weaken a fish's natural defences.

Treatment

Perform a large water change, add an aquarium salt (chloride) which helps block nitrite uptake, stop feeding, and dose beneficial bacteria. Test daily until nitrite reaches zero.

While treating, increase aeration, remove activated carbon from the filter (it absorbs medication), finish the full course even after the fish looks better, and keep the water immaculate with regular changes. Always dose for the tank's true water volume and follow the product instructions.

Prevention

Cycle tanks fully before stocking, stock slowly, and test regularly during the first weeks of a new tank. The golden rules are simple: quarantine every new fish for 2-4 weeks, never overstock or overfeed, avoid sudden temperature or chemistry swings, and do regular partial water changes. A stable, clean, low-stress aquarium prevents the large majority of disease.

FAQ

This information is general guidance, not veterinary advice. Consult an aquatic vet for serious cases.