Sick fish first aid: common symptoms, what they mean, and what to do first

Quick answer

When a fish looks sick, test ammonia and nitrite first. Poor water quality causes many "mystery" symptoms. If readings are off, do a partial water change and increase aeration. If water is good but symptoms persist, isolate the fish in a quarantine tank and treat based on the most likely cause. Avoid mixing medications without a plan.

Step 1: stabilize the water

Before you reach for medicine, confirm the basics:

• Ammonia: 0
• Nitrite: 0
• Temperature: stable and appropriate for the species
• Oxygen: good surface movement

If ammonia or nitrite is above 0, prioritize water changes and aeration.

Step 2: identify the symptom pattern

Use this quick symptom map to narrow down what is happening.

Gasping at the surface

Common causes:

• Low oxygen
• High ammonia or nitrite
• Temperature too high

Actions:

• Increase surface agitation or add an air stone.
• Do a partial water change.
• Check that the filter is running properly.

White spots (like grains of salt)

Common cause:

• Ich (often triggered by stress)

Actions:

• Move fish to quarantine if possible.
• Treat with an ich medication following the label.
• Keep temperature stable and increase aeration.

Frayed fins or fin rot look

Common causes:

• Poor water quality
• Stress or fin nipping
• Bacterial infection

Actions:

• Improve water quality and reduce stress first.
• Check for aggressive tank mates.
• Treat in quarantine if deterioration continues.

Bloated belly

Common causes:

• Overfeeding
• Constipation
• Internal issues

Actions:

• Stop feeding for 24 hours.
• Resume with smaller portions.
• Review diet and avoid constant high-protein foods.

Step 3: decide whether to isolate

Isolate in a quarantine tank if:

• Symptoms are spreading
• The fish is being bullied
• You plan to medicate

Treating the display tank can affect plants, invertebrates, and filter bacteria.

The biggest medication mistakes

• Mixing multiple medications "just in case"
• Treating without testing water
• Stopping treatment too early
• Using the wrong dose for tank volume

If you are unsure, go slow and keep water quality excellent.

FAQ

When should I treat the whole tank?

Only when multiple fish show clear symptoms and you cannot isolate. Otherwise, quarantine treatment is safer.

Can stress alone make fish look sick?

Yes. Poor water quality, bullying, or rapid changes can cause clamped fins, hiding, and loss of appetite.

What is the safest first move if I am unsure?

Test water, do a partial water change, increase aeration, and observe. Many issues improve with stability.

Next step

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