Open sores

A reddened open wound with a pale eroded rim and missing scales — an ulcerative bacterial infection that needs prompt attention. Compare your fish with the reference photo above and cross-check your logged parameters in Aquarium Tracker before medicating.

SymptomOpen sores

CausesBacterial infection entering through an injury or a parasite bite / Poor water quality suppressing the immune response

TreatmentImprove water quality immediately — clean, stable water is the foundation of recovery / Isolate the fish so the wound is not aggravated and treatment can be targeted

PreventionQuarantine new fish; zero ammonia & nitrite

What causes open sores?

Bacterial infection entering through an injury or a parasite bite
Poor water quality suppressing the immune response
Aggression or fighting causing repeated wounds
Rough handling, netting or sharp décor

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.
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

Improve water quality immediately — clean, stable water is the foundation of recovery
Isolate the fish so the wound is not aggravated and treatment can be targeted
Identify and remove the cause: aggression, sharp décor or an underlying parasite
Ulcers deep enough to expose muscle risk secondary infection and need professional input
Antibacterial treatment may be warranted, but confirm the diagnosis before dosing the display tank

Prevention

Keep ammonia and nitrite at zero — most bacterial and fungal problems follow a water quality lapse rather than arriving from outside
Remove sharp décor and address bullying; these conditions take hold on damaged tissue
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 open sores contagious to my other fish?

Possibly. The bacteria involved are usually present already and take hold where a fish is weakened, so healthy tank mates are at lower risk than the water quality suggests.

How quickly do I need to act?

Within a day or two. Open wounds invite secondary infection, so the window narrows the longer it is left.

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.