Red streaks in fins or body

Fine blood-red streaks through the fins or reddened patches on the body — often septicaemia, a bacterial infection in the bloodstream. Treat as urgent. Compare your fish with the reference photo above and cross-check your logged parameters in Aquarium Tracker before medicating.

SymptomRed streaks in fins or body

CausesSystemic bacterial infection, frequently following prolonged poor water quality / Ammonia or nitrite exposure damaging tissue and inviting infection

TreatmentTest ammonia and nitrite immediately and perform a partial water change if either is detectable / Verify your dechlorinator is being dosed correctly for the volume changed

PreventionQuarantine new fish; zero ammonia & nitrite

What causes red streaks in fins or body?

Systemic bacterial infection, frequently following prolonged poor water quality
Ammonia or nitrite exposure damaging tissue and inviting infection
Severe stress from transport, overcrowding or rapid parameter swings
Chemical irritation from overdosed medication or untreated tap water

What to check before medicating

What to measure	Typical target	What an off reading points to
Ammonia (NH₃/NH₄⁺)	0 ppm	Anything above zero means the biological filter cannot keep up. Toxic to fish long before it is high enough to feed algae.
Nitrite (NO₂⁻)	0 ppm	A reading above zero points to an immature or disrupted filter — common after a media change, medication, or a large clean.
Nitrate (NO₃⁻)	5–20 ppm planted · under 10 ppm reef	Climbing nitrate signals more waste going in than water changes and plants export. Near zero in a planted tank starves plants and favours algae.
Temperature	within your species range, steady	Warmer water holds less oxygen and speeds both metabolism and pathogen growth. Sudden swings stress fish more than a steady figure at either end.
pH	stable, within your species range	Stability matters more than the number. A drifting pH usually means depleted KH or a CO₂ swing rather than a pH problem in itself.
Stocking level and mix	suits volume, footprint and temperament	Crowding raises waste and stress together, which is why disease often follows a new addition rather than a parameter change.
Recent changes	note anything from the last 2–3 weeks	New livestock, plants, media, lighting or dosing are the usual trigger. The cause often precedes the symptom by a fortnight.

Take these readings first. A large share of what looks like disease is a water quality problem, and medicating a tank whose ammonia is climbing treats the symptom while the cause keeps working.

Treatment steps

Test ammonia and nitrite immediately and perform a partial water change if either is detectable
Verify your dechlorinator is being dosed correctly for the volume changed
Isolate affected fish where possible and keep conditions stable
Septicaemia progresses quickly and rarely resolves on water quality alone
Seek veterinary or specialist advice promptly — this often requires antibiotics

Prevention

Test ammonia, nitrite and nitrate on a schedule rather than only when something looks wrong
Keep parameters stable — sudden swings after a large water change cause more problems than the readings they correct
Avoid overstocking; crowding raises waste and stress at the same time
Feed only what is eaten in a few minutes, and remove the rest

Frequently asked questions

Is red streaks in fins or body contagious to my other fish?

The underlying infection can spread, particularly where fish are already stressed. Treat quickly and check water quality first.

How quickly do I need to act?

Within a day. Septicaemia indicates infection in the bloodstream and progresses quickly.

Should I treat in the display tank or a hospital tank?

A separate hospital tank is preferable where the fish is well enough to move: doses are easier to control, and many medications harm invertebrates, plants or the biological filter. Treat in the display only when the whole tank is affected or moving the fish would add more stress than the treatment removes.