Koi Fish Ammonia Gill Burn: Chemical Gill Damage and Gasping

Ammonia gill burn is chemical injury to the delicate gill tissue caused by elevated ammonia in pond water. Koi release ammonia as a normal waste product, and uneaten food, decaying plants, and sludge add even more. In a healthy pond, beneficial bacteria convert ammonia to nitrite and then nitrate. When that system is overwhelmed or disrupted, ammonia can rise to dangerous levels.

The gills are where koi take in oxygen and regulate salts and water balance. When ammonia irritates and burns this tissue, the gills may become inflamed, swollen, and less able to exchange oxygen. That is why affected koi often gasp at the surface, breathe rapidly, clamp their fins, or act weak.

This is not a single disease with one cause. It is a water-quality emergency that can happen in new ponds, crowded ponds, after filter crashes, or after sudden changes in pH. High pH is especially important because it increases the more toxic unionized form of ammonia.

Some koi recover well if the water problem is corrected early and secondary complications are limited. Others develop severe gill damage, stress-related infections, or sudden death. Your vet can help confirm whether ammonia is the main problem and guide the safest next steps.

The immediate cause is excess ammonia in the water, especially the unionized form that becomes more toxic as pH rises. In practical terms, that means a pond can become much more dangerous when ammonia is present and the water is alkaline. Warm temperatures can also increase stress and reduce oxygen availability, making breathing problems more obvious.

Common triggers include a new pond that has not fully cycled, overstocking, heavy feeding, dead organic debris, clogged filters, power outages, pump failure, or cleaning the biofilter so aggressively that beneficial bacteria are lost. Adding many new fish at once can also overwhelm the nitrogen cycle.

Sometimes the problem is not only ammonia production but also biofilter failure. Beneficial nitrifying bacteria convert ammonia into less harmful compounds. If those bacteria are immature, damaged by medications, disrupted by sudden pH swings, or reduced after major maintenance, ammonia can spike quickly.

Koi with ammonia burn may also have secondary problems. Damaged gills are more vulnerable to parasites and bacterial infections, and low dissolved oxygen can make the fish look even worse. That is why your vet may recommend looking beyond ammonia alone, especially if the pond history or exam findings do not fit a straightforward water-quality event.

Diagnosis starts with history and water testing. Your vet will want to know when signs started, whether multiple koi are affected, whether the pond is new, how many fish were added, what and how often you feed, and whether there were recent filter cleanings, medication use, storms, or power failures. Water testing usually includes total ammonia nitrogen, nitrite, nitrate, pH, temperature, and dissolved oxygen when available.

A physical exam may focus on breathing effort, body condition, mucus production, and gill appearance. Gill tissue affected by chemical injury may look inflamed, swollen, pale, dark, or coated with excess mucus. Because several conditions can mimic ammonia burn, your vet may also recommend gill or skin biopsies, microscopic evaluation for parasites, and sometimes bacterial culture or necropsy if deaths have occurred.

One important detail is that total ammonia and toxic unionized ammonia are not the same thing. Your vet may interpret the ammonia result together with pH and temperature to estimate how dangerous the reading is for koi in that pond. This helps explain why some fish crash at one ammonia level while others appear less affected.

If ammonia is high, treatment often begins while the workup continues. That is because delaying correction of the water problem can allow ongoing gill injury. Your vet can also help avoid overly aggressive changes that might worsen stress, especially in ponds with unstable pH.

Prevention centers on stable water quality. Test pond water regularly, especially ammonia, nitrite, pH, and temperature. Extra testing matters after adding fish, changing feed, cleaning filters, storms, medication use, or any equipment failure. A pond can look clear and still have dangerous ammonia.

Avoid overstocking and overfeeding. Feed only what your koi can consume promptly, and remove decaying leaves, sludge, and dead plant material before they break down. Keep pumps and biofilters running reliably, and avoid washing biological media in chlorinated tap water unless your vet specifically advises a different approach.

Quarantine new fish when possible and add them gradually so the biofilter can keep up. New systems are especially vulnerable during the first weeks while nitrifying bacteria establish. If you need to make corrections, controlled changes are usually safer than dramatic swings.

If your koi have had one ammonia event, make a written pond plan. Include normal test targets, feeding amounts, maintenance dates, and what to do during power outages or heat waves. That kind of routine often prevents repeat emergencies and helps your vet troubleshoot faster if problems return.