Why Do My Fish Keep Dying?

When an aquarium owner’s fish start getting sick and dying, one of the first reactions of most people is to run to the store to get medication to keep other fish from dying.  This will generally only make the original problems much worse, likely causing further fish losses.  Instead, the environment the fish are being kept in should be assessed to identify the root cause of the problem.  In the majority of cases, fish getting sick and dying are merely a symptom of a water quality or other problem in the tank, not a chance epidemic in your tank.

The primary reason most freshwater aquarium fish die is poor water quality.  The main causes of poor water quality are:

  • Uncycled tank
  • Insufficient filtration
  • Insufficient water changes
  • Insufficient removal of fish waste
  • Too many fish for the size tank
  • Contamination

Many people who are new to aquariums don’t understand how to identify water quality problems.  The three most important water parameters for freshwater aquariums are

  • Level of ammonia
  • Level of nitrite
  • Level of nitrate

Ammonia, nitrite and nitrate are nitrogen-based compounds that are produced by fish in the form of waste, but also by decomposing food, etc.  The best way to test the levels of these compounds is by using a master test kit, found at most aquarium stores.

Non-zero levels of ammonia and nitrite are indications of New Tank Syndrome, also called a cycling tank.  See the New Tank Syndrome section below.

Elevated levels of nitrate (>40-60ppm) is caused by Old Tank Syndrome, discussed below.

Symptoms Of Water Quality Problems

The major problem with water quality problems is that fish don’t recognizably die directly from poor water quality.  They die from opportunistic diseases that take advantage of a weakened fish, such as:

  • dropsy
  • internal parasites
  • bacterial infections
  • fungal infections
  • etc

New Tank Syndrome

“New Tank Syndrome” is the common name given when fish become ill or die in a newly establish aquarium.  Fish naturally produce ammonia as a waste product.  Excess fish food will rot and produce ammonia as well.  Aquariums containing fish need to establish a “cycle”, which is basically colonies of bacteria in the aquarium that process fish waste from poisonous ammonia and nitrite to less harmful nitrate.  Please see the page on cycling aquariums for more information.  In new tanks, tanks that have had the biological filter damaged or destroyed, or tanks that suddenly were overfed or had a big increase in fish, ammonia and nitrites will build up.  These compounds are quite poisonous to fish, and a build up of either or both ammonia and nitrite generally results in sick or dying fish.

If you suspect new tank syndrome, because the tank is new, a large amount of food was recently introduced, or many new fish were introduced, follow these steps:

  1. Stop feeding the tank.  Continuing to feed will make the problem worse.  Most fish can easily go several days without eating.
  2. Test the water parameters with a master test kit.  If none are available, take a sample of water to a local aquarium store, and they will generally test it for free.  This isn’t ideal, as many stores use unreliable “test strips”, but it should be accurate enough to show whether or not this is the problem.
  3. Perform partial water changes until the ammonia and nitrite levels get to zero and stay there.  The aggressiveness of the changes really depends on the level of ammonia or nitrites.  The higher the level, the more frequent and large the water changes need to be.  At 1 to 2ppm, change 50% of the water daily.  Above that, change 60% to 70% twice daily.

Old Tank Syndrome

Old tank syndrome happens in cycled tanks with well functioning bio-filters.  Over time, ammonia and nitrite are consumed by bacteria to produce nitrates.  Nitrates are far less toxic to fish than ammonia and nitrite, but in high concentrations, it will sicken or kill fish.  Unless some action is taken to reduce the concentration of nitrates, eventually old tank syndrome will set in and result in dead fish.

Old tank syndrome can be avoided with these precautions:

  1. Perform periodic water changes.  commonly 20% to 50% weekly, depending on the number of fish, amount fed, etc.  The target level for nitrates should be under 20ppm.  Using a master test kit and some experimentation will help you determine the right frequency and volume of water changes for your tank.
  2. Don’t overfeed fish.  If you see excess food laying on the bottom of the tank after feeding, you are most likely feeding too much.
  3. Vacuum the gravel periodically.  In tanks that have sand or gravel, over time fish waste and excess food get trapped and the decomposition process can become a serious problem for the level of nitrates.

If you suspect old tank syndrome, follow these steps:

  1. As with above, stop feeding the tank.  Continuing to feed will make the problem worse.
  2. Test the water parameters with a master test kit.  If none are available, take a sample of water to a local aquarium store, and they will generally test it for free.  This isn’t ideal, as many stores use unreliable “test strips”, but it should be accurate enough to show whether or not this is the problem.
  3. Perform partial water changes until the nitrate levels get below 20ppm.  The aggressiveness of the changes really depends on the level of nitrates in the tank.  The higher the level, the more frequent and large the water changes need to be.

Treating Illness with Medication

Many fish-keepers will attempt to treat the apparent disease with antibiotic, anti-parasite or anti-bacterial medications, but not the underlying cause – water quality.  Indeed, particularly in the US where antibiotics are readily available, many new-comers make a bad situation much worse by treating a tank with medication that kills off the colony of beneficial bacteria, leaving the tank uncycled.

It has been my experience that, in nearly all cases, fish that are showing obvious signs of illness will not survive despite best efforts and intentions.  To be sure, there are many astute and experienced fish-keepers who can nurse a sick fish back to health, but such experienced people also know that prevention is far more effective than a cure.

Good Practices For Maintaining Fish Health

Filtration

Ensure the aquarium has the proper amount of filtration.   While there are a number of variables, such as volume of water, number and types of fish, amount of food used to consider, the general rule of thumb is to size the filter to pump the water between 5 and 10 times per hour.  For example, a 55G aquarium needs filters that will pump between 275 and 550 gallons per hour.  My strong recommendation is to purchase two smaller filters whose flow rate adds up to 10 times the volume of the tank.  The That might seem excessive, but it does ensure good filtration and provides some ability to ride out the failure of one filter.

Using a UV filter is also a very good way of eliminating opportunistic microorganisms in the aquarium.

Water Changes

As with sizing filters, there is are a lot of variables that go into how much and how often to perform water changes.  Internet forums are full of recommendations… from 20% per month to 20% per week.  There is a sizable contingent of well intentioned people who insist that anything more than 20% per week is harmful to the fish.  I do not subscribe to that way of thinking – fish will not suffer from having too much clean water.  I perform 50% changes weekly on my tanks with good results.  It’s a lot of work, but the Python makes it manageable.

Removal of Fish Waste

Some fish waste is dissolved or suspended in the water and is removed through water changes.  However, far more of it hangs around in the form of poop at the bottom of the tank.  Tanks that have gravel on the bottom make this situation hard to see, and without a thorough cleaning of that gravel, bad things can begin to happen, even if the other suggestions are followed.  First, I would strongly recommend most people to skip the gravel, unless there is a particular need.  Spend the money on pieces of drift wood or similar things to cover the bottom.  These can be moved around when it comes time to clean.  Regardless, the fish waste needs to be pulled out weekly as part of the water changes.  A gravel vacuum, like the Python, can clean both bare bottom and gravel bottom tanks.  Bare bottom tanks have a distinct advantage in that it’s easy to see the waste and see that it has all been sucked out.  In gravel tanks, a thorough cleaning of all the gravel is needed to ensure getting most of the waste.

Too Many Fish

Again, there is not a 100% reliable method of determining a safe number of fish for a tank.  The rule of thumb often quoted for freshwater fish is 1 inch of fish (at it’s adult size) per gallon of water.  This breaks down, however, with large fish like goldfish, oscars, etc.  Fish that are packed in to an over stocked tank are very likely to contract a serious infection and die due to the combination of a compromised immune system from stress.

Proper Tank Size

Determining the right size aquarium for the fish you are keeping is as much an art as it is a science.  Fish that grow large, such as goldfish, koi, knife fish, etc, will experience similar symptoms to those when a tank is stocked with too many fish.

Getting Immediate Help For Your Specific Problem

There are several great forums on the Internet that have emergency fish help sections where questions are generally answered in minutes by very knowledgeable, friendly and caring people.  Two such forums are here:

If you are lost on how to proceed, or need validation on what you are doing, I strongly encourage you to post a message on one of those forums.  Be sure to read the link labeled “read this before posting” for which ever forum you visit.  Following that forums instructions on information to include in your post will make helping you much faster and easier.

2 comments to Why Do My Fish Keep Dying?

  • katwhipps

    can anybody help, i have a tropical fish tank, and all the fish r diein even sucker fish, i have checked water its perfect, done tank change and done treatment incase there is a dease is in the water, but they keep dieing! help!

  • admin

    What is “perfect”? What are the values for ammonia, nitrite and nitrate?

You must be logged in to post a comment.