Ammonia Spike: Causes and Emergency Fix

A high ammonia reading is an emergency. Learn to find the source, whether a dead fish, a crashed filter, or chloramine, and the immediate steps that protect your fish while the cycle recovers.

A high ammonia reading is one of the few true emergencies in fishkeeping, because ammonia is acutely toxic to fish. In an established tank ammonia should read zero, so any detectable level means something has gone wrong with the balance between waste production and the biological filter. Successful troubleshooting is part detective work, finding and removing the source, and part first aid, protecting the fish while the cycle recovers.

Why it is dangerous

Ammonia exists as un-ionised NH3 and ionised NH4+, and the NH3 form is roughly 100 times more toxic to fish. Critically, the toxic fraction rises with higher pH and temperature, so the same reading is far more dangerous in warm, alkaline water. Tissue damage can begin above about 0.05 mg/L of un-ionised ammonia; affected fish gasp at the surface with damaged gills. The next product, nitrite, is also toxic.

Common causes

• An uncycled or brand-new tank that has not yet grown enough nitrifying bacteria.
• A filter crash: media rinsed in chlorinated tap water, a power outage, or replacing the filter, all of which kill or remove the bacteria.
• A dead fish or snail, a rotting plant, or uneaten food decomposing unseen.
• Overstocking or overfeeding, so waste outpaces the bacteria.
• Disturbing a dirty substrate and releasing trapped waste.
• Tap water containing chloramine, which can register as ammonia, or a medication that has harmed the biofilter.

Diagnose the source

Test to confirm the reading, then investigate. Do a head-count: is a fish missing or hiding dead behind the hardscape? Look and smell for rotting matter, uneaten food or a decaying plant. Recall recent changes: did you clean or change the filter, add fish, overfeed, or change water source? The cause is usually a recent event.

The emergency fix

• Remove the source: take out any dead animal, excess food or rotting plant immediately.
• Do a large dechlorinated water change to dilute the ammonia directly; in a severe crash a change of 50 percent or more may be warranted.
• Dose an ammonia detoxifier as first aid to bind the toxic ammonia temporarily while you work.
• Stop or sharply reduce feeding until ammonia and nitrite return to zero.
• Add aeration, because toxicity worsens in warm water and good oxygen supports both fish and the bacteria.
• Help the cycle recover by seeding beneficial bacteria, and keep testing daily.