Avoiding Mystery Fish-Death in Marine Aquariums

Let's face it, sometimes fish die.  So does every other living thing on Earth.  But when your aquarium is healthy and thriving, the fish are vibrant, active, and eating- then suddenly you find dead fish- it can be extremely frustrating.  After over 25 years in the live fish business, we are here to help.  We would like to share with you some of the potential causes of "mystery fish death", and by doing so, hopefully save the lives of some fish, and lower the frustration level of current and future hobbyists.

I remember a very specific situation, back about 20 years ago when a very frustrated customer approached me and presented me with a puzzling quandry:  the fish keeper basically stated that every Saturday, she had a dead fish in her aquarium.  It was like clock-work, Saturday mornings were made for scooping dead fish.  Of course the first thing we did was to perform a complete water test.  No ammonia, no nitrite, low nitrates, perfect pH, perfect salinity- none of the normal telltale causes of fish death. We inquired about the tank's temperature, the condition of the filtration equipment, etc. etc...  Was something unplugged, were the fish gasping, did they have disease symptoms, no no no was always the answer- this was becoming quite the mystery.  After a long Q & A session, something became evident:  none of the normal aquarium problems were evident- the cause of fish death was definitely due to something else.  When this occurs, and after evaluating all normal risk factors and potential causes, we narrowed it down to one thing: POISON.  

"Burning scented candles can release toxins in to the air that can kill aquarium fish"

"Every Friday i cook fish for dinner.  My husband would always spray air freshener in the kitchen to get rid of the fishy smell that lingered after dinner."  BINGO!!  Air freshener sprayed in the room on Friday nights, dead fish Saturday mornings.  I think we just discovered our cause of death.  Air fresheners sprayed near aquariums can get into the water and kill fish.   So although your air "smells fresh," what you are really smelling is chemicals in the air.  Chemicals, that when dissolved in water can become toxic to aquatic life.   And its not just air freshener, we've solved many other mystery fish death issues for other clients, and narrowed it down to similar toxins coming from household items:  Febreze, carpet & upholstery spray, spray paint fumes, scented candles, glass cleaner, Glade plug-ins, etc. etc.....  These household items are killing fish and leaving their owners discouraged and bewildered.   


  • WINDEX or any other commercially sprayed glass cleaner
  • FEBREEZE or any similar carpet or upholstery spray
  • GLADE PLUG-INS & all automated scented spray devices
  • SCENTED CANDLES, especially potent in small rooms
  • PERFUME, lotions, etc.  
  • GASOLINE or Exhaust fumes
  • METAL plumbing fittings can be poisonous to aquarium life, use NO metal pipe fittings anywhere that your aquarium water flows, high dissolved metal content becomes toxic to marine life.
  • HEATING SYSTEM fumes & vapors from wood burning devices or locating aquariums too close to oil burners & other sources of combustion
  • CONCENTRATED cigarette smoke
  • SOAP.  on you hands or in a bucket used for aquarium water = toxic
  • Any and all types of aerosol sprays and chemical pump sprays that create vapors in the air could become a toxin to fish and cause fish death
  • Overdosing of aquarium-safe products can also create an unsafe condition that can kill fish, especially Red Sea NO3-PO4-X -- If overdosed becomes toxic to fish
  • Kitchen sponges.  Never use them in your aquarium because they are treated with strong anti-microbial chemicals that are toxic to aquatic life.


  • Evaluate.  Did only 1 fish perish?  or a whole tank?  If all fish appeared and acted healthy then all die suddenly, this is almost always due to environmental poisoning.
  • Look for disease signs.  Cloudy eyes, gasping, white spots, etc. are all signs of disease.  Treat accordingly.  Fish will usually exhibit disease symptoms for days if not weeks before perishing.  
  • Test the water.  HIgh ammonia or nitrite levels, incorrect pH levels, and extreme temperatures can kill fish.  Act accordingly to correct issues with the water chemistry.  Aqua Dreams offers free in-store water testing, analysis & trouble shooting.
  • Check equipment for failure.  Pumps, heaters, aerators, etc.  A defective heater can cause water temp to drop or skyrocket to unsafe levels.
  • Look at the tail of the dead fish, is it jagged and chewed up? - a telltale sign that you have an aggressive fish that could be killing your other fish.
  • Ask family members if any one has sprayed or used in chemical products in or around the aquarium.  Educate household members that most household products are NOT safe for aquariums and fish.
  • Check buckets & nets for signs of soap or other chemical residues, label buckets "for aquarium use only" and never use them for soap or cleaners.
  • Be sure to avoid hand lotions & soap prior to placing your hand in the aquarium water or handling fish foods that will go into the tank- it's a good idea to rinse your hand / arm with warm water before "going in".  
  • Check for scented candles burning in the room where the aquarium is located, avoid them if possible.
  • If you determine that a toxin or posion has potentially entered your aquarium, the best thing to do is change a large portion (50%) of the aquarium water ASAP and add Fresh activated carbon or other absorbent resin or filter material to reduce the levels of toxin.  

"Spray cleaners, kitchen sponges, and metal pipe fittings should not be used in or around aquariums"

We hope this article has been helpful and informative.  Avoiding fish loss due to household toxins should become an "obvious" for most serious hobbyists.  We hope that educating you will make you a better hobbyist.  Please share this article with other fish keepers, in the hopes that precautions are taken to avoid the accidental poisoning of your aquarium fish in the future.