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Freshwater Aquarium Fish Diseases & Prevention

Have you ever woken up to find your fish sick or dead? Do you know the causes? Do you know how to identify different fish diseases? Do you know how to prevent or treat various fish diseases? This article will give you the answers.


Why do fish get diseases?

People get sick for multiple reasons, and so do fish. There are many factors that make fish suffer from diseases.



Each fish tank has its own density. If you keep too much fish, they will compete for food and form two levels of differentiation. And the water environment will deteriorate, which will cause oxygen deficit and massive outbreaks of disease.


Improper feeding

Feed fish with rotten, spoiled, or infested food. Feed them with the same bait for a long time, causing malnutrition. Use an improper way to feed fish and leave too much residual bait in the water.


Improper treatment

Use improper catching methods or behave roughly when ship and breed fish, change water or invert the tank. These will scrap off the surface mucus and scales of the fish, and then cause fish diseases.


Use unsterilized tools

Fish farming tools carry a lot of germs, parasites and pests. Use unsterilized tools will result in fish disease infection.


Inappropriate water temperature

Fish are cold-blooded creatures, their body temperature varies with water temperature. When the water temperature changes suddenly, the resistance of fish may decline because they can not adapt to the temperature change.


Some common diseases in fish

Fish would suffer from many diseases, the following paragraph will introduce the causes and symptoms of some common diseases in them.


White spot

  • Cause

Large temperature difference, ichthyophthirius multifiliis parasite

  • Symptoms
  1. At the beginning, fish will rub their bodies against hard objects to get rid of the pathogens.
  2. Many small white spots can be seen on their body, fins and gills.
  3. Diseased fish are emaciated, floating on water or clustered in a corner, rarely moving.
  4. With time passing by, their body will seem like being covered with a layer of white film, the mucus will increase and the color of the body will be dim and dark.



  • Causes

The buildup of fluid inside the body cavity or tissues.

  • Symptoms
  1. The abdomen, eyes, parts near the anus begin to swell.
  2. Scales start to point outward instead of lying flush with their body, showing a "pine cone" appearance.
  3. Fish that suffer from dropsy will lose appetite, energy, movement and color in their gills.
  4. A curved spin.


Velvet / Rust / Gold Dust

  • Causes

Dinoflagellate parasites of the genus Piscinoodinium, pathogens carried by unquarantined fish.

  • Symptoms
  1. Swim abnormally.
  2. Rub their bodies against hard objects to relieve their discomfort.
  3. Many small yellow or white spots can be seen on their body.
  4. A lack of appetite spirit and energy.
  5. Lose weight.



  • Causes

Eat unclean food or eat too much, blocked excretion, gut being infected by bacteria.

  • Symptoms
  1. Swollen abdomen, showing erythema, prominent anus.
  2. Stay in the corner of the aquarium, a short period of body muscle convulsions.
  3. A lack of appetite.
  4. Produce white feces.


Fin rot

  • Causes

Poor water quality, stimulated by new water, get hurt.

  • Symptoms
  1. Scales on the fin fall off, bone of the fin exposed, sometimes there will be pale yellow liquid on the fin.
  2. Swim slowly and usually stay alone.
  3. Lack of appetite, lose balance.


Swim bladder disorder

  • Causes

Intestinal parasites, constipation induced by over feeding, low water temperature. 

  • Symptoms
  1. Have difficulty controlling their ability to float or sink.
  2. Swollen abdomen or curved back.
  3. Lack of appetite.


Cotton mouth

  • Causes

Gram-negative, aerobic, rod-shaped bacterium Flavobacterium columnare. 

  • Symptoms
  1. Frayed and ragged fins, followed by the appearance of ulcerations on the skin, and subsequent epidermal loss, identifiable as white or cloudy, fungus-like patches.
  2. Accumulation of mucus on the gills, head and dorsal regions.
  3. Color change of gills, either light or dark brown.
  4. Rapidly and laboriously breath.


How to keep your fish away from diseases?

The best way to keep our fish healthy is disease prevention rather than cure.

There are some suggested methods for disease prevention.

1. Keep water clean

Fish get disease mainly because of the poor water quality, so the most important thing is to ensure water quality.

To get crystal-clear water, aquarium filter would be a good choice. There are different kinds of filters on sale, you can choose the most suitable one for your fish tank.

You can also do a water change to maintain good water quality. It can be done once a week or every two weeks according to the types and the number of fish you are keeping and the how big your tank is.


2. Adjust water temperature

The water temperature should change according to the fish you keep, the weather and the place you put your tank and so on. For example, if you are keeping a tropical fish, you should heaten your water, and if you are keeping a cold fish, you should lower the temperature.

To adjust the temperature, you’d better buy an aquarium heater, which can help you meet the temperature demands of fish.


3. Avoid overstocking

As we mentioned before, overstocking may cause fish diseases, thus we must avoid doing so. But how to figure out the amount and size of fish to keep in a certain tank?

There is a calculation here. Let me take a 60*45*45cm fish tank for example, assuming the water level is 40cm high, then ignore the glass premise, the water volume=60*45*40/1000=108L.

Generally for small and medium size fish, average 1L water to keep 1cm fish, this is a safer range. Take the above tank as an example, the water volume is 108L, then the total length of the fish in the tank (excluding the tail fin) should be no more than 108cm.

The density of fish in a fish tank is as follows:

For every 2.5 cm long saltwater fish, 300cm² of water surface area is needed; for cold water freshwater fish, 190cm² is needed; and for tropical freshwater fish, 75cm² is needed.


4. Feed scientifically

Make sure the bait you feed to your fish is fresh and clean. Usually change the type of bait to ensure your fish is getting variety nutrients. Timely remove the residual bait to avoid water pollution.


5. Be gentle

When you have to breed and ship the fish or you need to change the water for it, be gentle. Every rough behavior has the potential to cause damage to the fish. The damage of fish’s skin can lower their immunity and allow germs to attack them, thus  causing diseases.


6. Use sterilized tools

Disinfection should be done before and after using tools to avoid external germs attacking your fish.


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