Why Fish Are Dying in an Aquarium


Variables from the source of your aquarium pets to the quality of your water can adversely affect the health of your fish. Some fish deaths are difficult to prevent, but you can bolster the lifespan in your aquarium community by following best fish-keeping practices.

Aquarium Maintenance

Without an aquarium kept clean through adequate filtration, regular water changes and frequent vacuuming of waste and food debris from the substrate, your fish are basically swimming in a big septic tank. Clean, circulating water helps keep up the natural defenses of fish so their immune systems can fight off potential bacterial, parasitic and fungal infections.

Conversely, you may be trying to keep the environment extra clean but stripping good bacteria in the process, or not cycling a tank before use. Siphon waste out of the gravel instead of rinsing rocks in order to keep good bacteria present. Consider using two filters so that you can change one to a fresh cartridge while the other ensures the cycle of good bacteria isn't disturbed.


  • Treat all tap water with dechlorinator before adding to an aquarium, don't use cleaning solvents or detergents to scrub the environment and rinse with hot water -- no soap -- before dipping your hands in the aquarium.

Bad Stock

Your flailing fish could simply be a matter of acquiring bad stock. Be sure you're buying from a store with clean, healthy tanks, and don't buy from tanks containing dead fish. Quarantine new arrivals for 4 to 6 weeks in a separate tank to avoid potentially infecting your existing crew of fish.

Know that some problems may show up later that can be traced to bad breeding stock and not necessarily the quality of your care, such as dojo loaches growing up crooked.

Illnesses That Fester and Spread

Your fish could be dying simply because you're not paying attention or acting fast enough at the first signs of illness.

Look for signs of illness including:

  • sores
  • loss of appetite
  • frayed fins
  • white fuzzy growths
  • redness around the gills or vent
  • lethargy
  • resting in an unusual place

Overfeeding or Underfeeding

You may not be feeding your species the right amount of food. Dumping too much food in a fish tank fouls the water with uneaten food and an abundance of waste, leading to overall poor health. Be sure to take into account the size of your fish, how they compete for food and whether you're feeding vegetarian, meat-eating or omnivorous species.

Only feed your fish as much as they can consume within five minutes, with a longer time allowance for shy bottom-feeders to come eat sinking wafers.

Tank Overcrowding

Some people may use the one-inch-of-fish-per-gallon rule and not take into account that they're buying juvenile fish who still have plenty of growing to do or full-bodied fish that will take up more overall area. What you're weighing is not just how much room a fish has to swim and turn around, but how much it fouls the water with its waste.

At the same time, you want to make sure that fish accustomed to schooling together aren't understocked. Use an aquarium calculator to determine if your stocking is appropriate for your tank size and filtration.

Dangerous Decorations

Sharp edges of plastics plants can tear delicate fins, potentially leading to infection. Putting items in the tank not specifically indicated for aquarium use, such as painted pottery, can cause trouble. Cute aquarium ornaments can also be death traps if a fish fond of tight spaces is able to squeeze in but not get out.


  • Fine sand is great for loaches who want to bury themselves, but a gravel vacuum inserted in the substrate can inadvertently injure or kill a bottom-dweller. Gravel vacuums should also have safety filters to avoid sucking up tiny fish.

Aggressive Tankmates

If the wrong species are mixed in a fish tank, some of your pets may end up as another's supper. An aquarium calculator will let you know if dangerous incompatibility exists. Weaker fish may also get bullied out of feeding time and end up starving to death. Fins or scales can be viciously nipped, causing injuries and infections.


  • Compatibility issues can also kill when fish of different water and temperature needs are housed together.

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