Nitrite in Shrimp Ponds — After Ammonia Comes Nitrite

Ammonia comes down, you breathe out, and a few days later shrimp start dying again. The aerators are running, dissolved oxygen reads 6 mg/L, and yet the shrimp come up to the surface. This is when you check nitrite. ■ Nitrite forms where ammonia passed through Nitrogen breaks down in two steps. Ammonia becomes nitrite, then nitrite becomes nitrate. Different bacteria run each step, and the second group settles in later than the first. So a window opens a few days after ammonia falls, where nitrite climbs. That is the normal sequence. The mistake is stopping your measurements because ammonia looked solved — the second accident happens in that gap. ■ Why it kills — oxygen is there, they cannot use it Nitrite interferes with oxygen transport in shrimp blood. However much oxygen is in the water, it does not move through the body. That is why shrimp show oxygen-starvation behaviour at normal dissolved oxygen readings when nitrite is high. If your aerators are at full and shrimp still come to the surface, do not add more air — measure nitrite. ■ The target — under 1 mg/L For white shrimp, keep nitrite below 1 mg/L. The safety margin narrows sharply with salinity. In low-salinity ponds, damage is reported at far lower concentrations. If you run around 5 ppt, hold yourself to a stricter number. ■ When it rises Newly started ponds, because the nitrifiers are not established yet. Right after disinfection, because you killed the useful bacteria along with the pathogens. Late in the cycle, as feed and waste raise the nitrogen load. And, as described above, a few days after an ammonia event. ■ When it rises — in this order First, cut the feed. Less nitrogen going in is what brings the number down. It is the fastest and surest move. Second, hold oxygen higher than usual. Nitrite is blocking oxygen inside the animal, so the water needs more of it. Watch the pre-dawn reading in particular. Third, consider a water change. Do not swap more than 30 percent at once — do 20 to 30 percent in stages, and measure the incoming water's temperature, salinity and pH against the pond first. Fourth, probiotics are prevention, not emergency medicine. Nitrifiers take days to weeks to establish. Nothing you add today lowers today's reading. ■ Records matter more than thresholds For a week after ammonia falls, check nitrite every day. With that record, the next cycle tells you roughly when nitrite will climb. Losses look sudden, but on ponds that keep records there is almost always a warning first. Shrimp365 logs ammonia and nitrite together so you can see the second accident coming. Free to use.