By Paul Curtis
Aquafinn / MinnFinn

Some fish swim a lot, some lay on the bottom, some will hang near the surface and some in the middle of the tank. Your fish are very good at telling you what they need. If their behavior changes from normal, it is our job to figure out what is going on. If you suspect something is going on, go through the steps below to help guide you on what may be going on.

Interactions With Other Fish

Selecting compatible fish for your tank can sometimes be a challenge. Temperaments vary, so take care in your fish selections. Putting the wrong fish together can be a disaster and cause a rapid deterioration in your fish so don’t be afraid to ask questions first.

Aggressive behavior in the tank can lead to bullied fish having its health affected. Stress is a powerful force and will have undesirable effects. There are sometimes exceptions to social rules with fish so always keep your eye on things.


  • Fin nipping and erosion
  • Scale loss and lesions
  • Fish hiding in remote parts of the tanks


Injuries are more common than you would think and often arise from interactions described above. But fish can also become injured from getting stuck in the tank furnishings, they can rub on abrasive objects causing damage, or can hurt themselves jumping or running into something. Injuries can lead to wounds and infections. 


  • Sores and scale loss on fish from scratching

  • Wounds from jumping

  • Wounds from getting caught in decorations or filtration

Water Quality

One of the most important tasks with your first tank is learning about water quality. What is it exactly, how does it affect my fish, and what do I need to do to achieve a healthy balance?  One problem with big changes in or inadequate water quality is you can’t see it happening.  We must now learn how to test for the major water quality parameters, such as ammonia and pH, so we can monitor the balance in our system.

If things start to change and the parameters become toxic to the fish, you will start to notice changes in your fish.  Behaviors and appearance can change and lead to fish getting sick and dying.


  • Gasping at the surface

  • Fish are lethargic

  • Fish breathing heavily

  • Fish more easily frightened
  • Dead fish that look normal


    • Toxicity from ammonia and nitrite
    • pH out of acceptable range
    • Temperature out of acceptable range
    • Oxygen levels inadequate


There are numerous choices of fish food:  flakes, pellets, frozen, dried, even new innovative gels.  What we feed is very important to fish health. Unlike our dogs and cats, we don’t have a good understanding of the basic requirements of the hundreds of different species available for the home aquarist. That makes it even more difficult to know what to feed.

With incorrect feeding, fish will not flourish and can develop abnormalities and/or get sick. A proper diet will provide a good balanced nutrition and usually allow your fish a long healthy life.


  • Slow growth

  • Deformities

  • Get sick easily

  • Dead fish that look normal 


    • Toxicity from spoiled food
    • Vitamin deficiency from old food
    • Poor quality feed
    • Improper food size
    • Improper food ingredients

Disease (Pathogen and Non-Pathogen Related)

Even the most seasoned aquarist will have to deal with disease now and then. The experience comes in recognizing when a problem is coming on early and then how to resolve the issue. A disease outbreak is usually the result of a change in the balance of interactions between the fish, environment and the pathogens in the system.  All of the topics addressed above can contribute to problems when any are out of balance or out of tolerance for the species you have.


(Internal And External)

  • White tufts or patches on gills, fins and/or body
  • Open sores on body
  • Fish getting skinny
  • Fish with distended abdomen
  • Fish with scales sticking out

(Mostly External)

  • Fish look lighter in color
  • Fish look darker in color
  • Fish are lethargic
  • Fish breathing heavily
  • Fins are closed
  • Fish sitting on bottom
  • Fish sitting close to water inlet
  • Fish with white spots like salt
  • Fish look slimy
  • Foam on surface of the tank


(Mostly External)

  • Fish look lighter in color


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