Indications a fish was caught with Cyanide

  • Fish caught with cyanide often do well for a few weeks but have had irreparable physiological damage which eventually catches up.
  • These fish usually stop eating and go to a corner and breath hard after having been eating well and looking good in your aquarium.
  • Prevalence of cyanide caught fish in the aquarium trade has gone down significantly in the past decade however you occasionally see them through certain supply chains and therefore in certain stores. Be weary of the following species
  • Fish that are commonly found in the trade having been caught with cyanide: dwarf angelfish such as the bicolor, coral beauty, rusty, and keyhole angels
    • Butterfly fish and tangs found in the Philippines are also commonly caught with cyanide
  • Cyanide toxicity or starvation however can be difficult to differentiate in a fish coming to a tank where the previous inhabitants are all immune to a resident parasite ( such as uronema and sometimes even brooklynella which both can be opportunistic) and the new fish gets the disease
  • With severe cyanide toxicity the gills appear brown. A veterinarian can look at blood under the microscope and diagnose that a fish has methemoglobinemia, the condition of red blood cells not being able to carry oxygen caused by cyanide poisoning.

Indications a fish has nitrite toxicity:

  • Fish is rapidly breathing and gills appear brown

  • Fish was recently in a long transport or is in a tank which is currently cycling

  • Fish was/is in a system confirmed to have above 1 ppm nitrite in freshwater or 2 ppm nitrite in saltwater (nitrite is not absorbed as well through the gills of fish in salt water because it is displaced by the chloride ion)



The drug which may reverse the effects of cyanide toxicity and nitrite poisoning  is methylene blue. It allows blood to be reduced from methemoglobin back to hemoglobin and restores to an extent the oxygen carrying capacity. Pure oxygen can be bubbled as a supplement to treatment to potentially increase total oxygen being carried by the fishes blood.

If the disease is not truly cyanide poisoning, or even if there is slight cyanide toxicity with a resulting infection you may need a microscopic examination for a diagnosis, or if that is not possible you need to treat the fish empirically (using rule out methods) such as with copper or formalin. Hopefully you can catch the fish and hopefully you have a quarantine tank (though if you had and used a quarantine initially you are much less likely to be in this predicament)