Ammonia (NH3  NH4) Poisoning in Aquarium

Ammonia – colourless, pungent gas and the primary component of fish wastes. It is harmful to fish and tends to rise in newly established aquaria due to a lack of nitrifying bacteria, adding too many fish at once, overfeeding, or a combination of these factors. It can also rise in heavily populated tanks due to generous feeding and/or insufficient filtration.

In water, ammonia exists simultaneously as free ammonia (NH3) and ammonium ion (NH4 +). Aquarium water test kits usually measure NH3  and NH4 combined. Ammonia is more toxic at higher temperatures and pH levels above 7.0, and less harmful at lower temperatures and pH levels below 7.0.

Causes of Ammonia Poisoning

  • Chemically treated tap water – using chemical called chloramines
  • Decomposition of organic matter – aquarium plants, fish excrement, and uneaten fish food
  • Overfeeding


  • Large volume water change
  • Lowering the pH of the water
  • Restrict feedings

How to measure Ammonia in Aquarium?

Nitrite Poisoning in Aquarium

Nitrifying bacteria living in the filter and aquarium convert ammonia to nitrite (NO2) (colourless and odourless). A rise in nitrite usually follows an ammonia spike. Nitrite inhibits oxygen exchange to fishes’ bloodstream, thereby causing them to suffocate.

Nitrite poisoning is also known as “brown blood disease” because the blood turns brown from an increase of methaemoglobin. It renders the blood unable to carry oxygen, and the fish can literally suffocate even though there is ample of oxygen present in the water


  • Fish gasp for breath at the water surface
  • Fish hang near water outlets
  • Fish are listless
  • Brown gills
  • Rapid gill movement


  • Large volume water change –  reduces NO2 content in water
  • Add Aquarium Salt  [1 table spoon for 1 gallon water] – blocks the nitrite absorption through the fish’s gills
  • Increase aeration – provides ample of oxygen saturation in the water
  • Restrict feedings – avoids production of additional NO2 into water

Prevention Tips

  • Stock new tanks slowly
  • Feed sparingly and remove uneaten food timely
  • Change water regularly
  • Test water regularly to detect problems at early stages

How to monitor Nitrites (NO2) in Aquarium?

NOTE: A spike up in ammonia will cause a spike up in nitrite as nitrite is a byproduct of partial oxidisation of ammonia by bacteria. Therefore, the prevention tips are applicable for both types of poisoning in aquariums.


Adams, C., 2021. The Nitrogen Cycle: Simple Step by Step Guide For Beginners. [online] Modest Fish.  [Accessed 31 March 2021].