How to Choose Healthy Aquarium Fish

Updated on July 19, 2019
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InterestCaptured is currently the curator at an aquarium design studio and has many years of experience and passion in the pet industry.

Learn how to recognize the signs of disease and stress in fish.
Learn how to recognize the signs of disease and stress in fish. | Source

One of the most important aspects of a healthy aquarium is making sure your aquatic newcomers are healthy and fit for the trip to their new home. Doing this can be as simple as watching fish before you buy them, as long as you know what to look for. This article is aimed at teaching you how to use observation to purchase healthy fish and avoid bringing diseases into your home fish tank.

How Healthy Fish Behave

As obvious as it may sound, it really helps to understand the behavior of a healthy fish before you can start to identify sick or stressed ones. You can begin by doing a bit of research on the fish you want to bring home before you bring it home. There are a number of reputable websites (liveaquaria, fishchannel, etc.), books (Adventurous Aquarist Guides, Compass Guides) and of course your local fish store staff that are all valuable resources when it comes to learning about these creatures. For the most part, a healthy fish should:

  • Eat when presented with food, not seem disinterested or spit food out after trying it.
  • Have bright coloration; pale or abnormally dark fish are usually stressed or diseased.
  • Swim actively with a symmetrical gait. An odd swimming pattern is a sign of poor health.

If you are not sure if a fish you are about to buy is eating, just ask to see it eat! Any reputable aquarium store should be more than willing to feed the fish in front of you. Even if a fish shows all of these signs of health, when buying new fish, there are a few things you always want to look for before making any hasty decisions:

Signs of Disease

Sometimes it is difficult, if not impossible, to detect a fish disease, but being a good observer goes a long way towards prevention. I can’t mention enough how important it is to understand the “normal” behavior of a fish before jumping to the conclusion that a fish is sick, but here are some general telltale signs that a fish may have health problems:

  • Sunken stomach (a gut infection or poor nutrition)
  • Cloudy eyes
  • Protruding eyes (eye damage or a bacterial infection)
  • Gasping (gill infection or ammonia/nitrite burn)
  • Bloating (dropsy or constipation—yes fish can get constipated!)
  • Protruding scales (looking like a pinecone, a sign of the disease dropsy)
  • Red streaking or blotches (often an blood infection or internal bleeding)
  • White spots (external parasites)
  • Black spots (external parasites)
  • Discoloration (high stress levels)
  • Excessive mucous production (external irritant)
  • Ulcers/lesions
  • Puffy white or grey growths (fungal infection or lymphocystis)
  • Erratic swimming (swim bladder issue, neurological disorder, or cyanide fished)
  • Lethargy
  • Inability to maintain buoyancy (swim bladder issue or heavy sickness)

White spots characteristic of the disease Cryptocaryon irritans.
White spots characteristic of the disease Cryptocaryon irritans. | Source

Signs of Stress

Much like it affects us, stress can pretty much send a fish to the grave. Stress can come from sources such as aggressive tankmates, improper water quality, not enough places to hide, too small of a tank, incorrect diet, or stray electrical current. These stressors weaken a fish’s immune system and leave the fish susceptible to infection, so never buy a noticeably distressed fish. Some visuals that signal a stressed fish include:

  • Reduced appetite (being aggressed upon, medicated, or improper foods)
  • Hiding behind heaters or under filters (usually means they're being aggressed upon)
  • Abnormally dark coloration
  • Abnormally light coloration
  • Clamped fins (often a bacterial infection)
  • Tattered fins (being picked on)
  • Missing scales (being picked on)

Scales missing from a fish, likely due to aggression from tank mates.
Scales missing from a fish, likely due to aggression from tank mates. | Source

Once you get used to looking for these signs when you purchase fish, you can start to determine which stores take consistently good care of their animals. We hold all of our fish for at least a week to proactively treat them for diseases and make sure they are eating well, so you know your fish is full of vitality and ready to go to its new home!

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

Questions & Answers


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      • Joan King profile image

        Joan King 

        13 months ago

        Unfortunately you often only discover the fish is unhealthy after you bring it home, These are good tips for ensuring the fish is in good health before brining it home.


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