Diagnosing and Treating Sick Fish

My Fish Looks Sick — What Should I Do?

Many fish diseases can look similar. Before reaching for medication, check the aquarium itself and look closely at the fish's symptoms.

First: Check the Water

Poor water quality can cause many of the same symptoms as disease and can make an existing illness much worse.

Ammonia Should be 0 ppm
Nitrite Should be 0 ppm
Temperature Verify it is appropriate
Saltwater Verify salinity
Freshwater testing: API Freshwater Master Test Kit
Do not medicate just because a fish is acting differently. Hiding, reduced appetite and temporary stress can occur after transport or introduction to a new aquarium. Look for persistent or worsening symptoms before choosing a treatment.

What Are You Seeing?

Use the visible symptoms below as a starting point. They can help narrow down the likely problem, but several diseases can produce similar signs.

White Spots / Salt-Like Dots

Small white spots combined with scratching, clamped fins or increased breathing may indicate an external parasite such as Ich.

  • Check water quality first.
  • Observe other fish for similar spots.
  • Treat the entire affected fish system, not only the visibly sick fish.
Freshwater: Aquarium Solutions Ich-X

Flashing / Scratching

Fish repeatedly rubbing against rocks, substrate or decorations can indicate irritation from parasites or poor water conditions.

  • Test ammonia and nitrite first.
  • Look closely at the skin and gills.
  • Check for white spots, excess mucus or rapid breathing.
Possible external parasite treatment: Seachem ParaGuard

Weight Loss / Stringy Feces / Wasting

A fish that continues losing weight despite eating, develops a sunken belly, or produces persistent abnormal feces may have an internal parasite problem.

  • Observe appetite and body condition.
  • Look for a pinched or hollow abdomen.
  • Internal parasites are especially common in newly acquired fish.
Wet Pets recommendation: Fritz ParaCleanse · PraziPro

Fin Rot / Sores / Popeye / Redness

Damaged fins, open sores, swollen eyes, redness, tissue loss or worsening wounds can be signs of bacterial infection.

  • Correct poor water quality first.
  • Check whether aggression or an injury started the problem.
  • Move the fish to a hospital tank when practical.
Recommended options: Maracyn Two · Seachem KanaPlex

Rapid or Heavy Breathing

Heavy breathing is a serious but nonspecific symptom. It can result from ammonia, low oxygen, temperature problems, gill irritation or parasites.

  • Immediately test ammonia and nitrite.
  • Increase surface agitation and aeration.
  • Verify temperature.
  • For marine fish, inspect carefully for signs of external parasites.
Test the aquarium before choosing medication.

Bloated / Swollen Abdomen

Swelling can have several causes including diet, internal parasites, infection or organ problems.

  • Observe whether the fish is eating and passing waste.
  • Check for raised scales or other signs of systemic illness.
  • Avoid repeatedly feeding a severely bloated fish.
If internal parasites are suspected: Fritz ParaCleanse

Saltwater Fish: White Spots, Dusting or Rapid Breathing

Marine Ich and marine Velvet require a different approach than most freshwater parasite treatments. When a marine fish is suspected of having one of these parasites, treatment should generally be performed in a separate quarantine or hospital aquarium.

Wet Pets Marine Treatment Option
Fritz CopperSafe

Copper concentration must be measured and maintained at the proper therapeutic level throughout treatment.
CopperSafe Dosing Calculator
NEVER ADD COPPER TO A REEF DISPLAY. Copper medications such as CopperSafe should not be used in an aquarium containing corals, shrimp, snails, crabs or other invertebrates. Avoid treating marine fish with copper in the main reef aquarium.

A Hospital Tank Is Often the Best Place to Treat

A simple separate aquarium makes it easier to observe the fish, control medication dosage and avoid exposing healthy livestock, plants, corals or invertebrates to unnecessary medication. Provide filtration, appropriate temperature and strong aeration during treatment.

When Using Medication: Follow the manufacturer's dosing instructions and complete the recommended treatment course. Many medications require activated carbon, Purigen or other chemical filtration to be removed during treatment. Increasing aeration is also a good practice with many fish medications.

Wet Pets Medicine Dosing Calculator  ·  View Aquarium Medications
Get help quickly if you see:
  • Severe or rapidly worsening breathing difficulty
  • Loss of balance or inability to remain upright
  • Rapidly spreading sores or tissue damage
  • Multiple fish becoming sick at the same time
  • Sudden unexplained fish deaths
  • A marine fish showing very rapid breathing with a fine dusty coating

This guide is intended to help narrow down common aquarium fish problems and is not a definitive diagnosis. Different diseases can produce similar symptoms, and medication should be chosen based on the complete picture: water quality, species, recent additions and visible symptoms. If you are unsure what you are seeing, take clear photos or video and your water test results to Wet Pets Aquatics before beginning treatment.