Hemorrhagic septicemia in fish: red streaks, red blotches and what to do

Hemorrhagic septicemia is a bacterial infection that has spread into a fish's bloodstream. It shows up as red streaks in the fins, red blotches or bleeding under the skin, and often sores, bulging eyes or a swollen belly. It is serious and can kill quickly, so isolate the fish, fix the water and contact a vet today.

It is usually caused by bacteria that are always present in aquarium water, mainly Aeromonas and Pseudomonas. They only take hold when a fish is weakened by poor water, injury or stress, so treating the fish without fixing the tank rarely works.

How to identify it

Red streaks in the fins and tail, like tiny burst blood vessels, often running along the fin rays.
Red blotches or bruising on the body, belly, gill area or around the mouth.
Red or bloody eyes, sometimes bulging (popeye).
Ulcers: open red sores, often with scale loss around them.
Red, swollen vent (where poop comes out).
Swollen belly or raised scales like a pine cone. See dropsy.
Behaviour: lethargy, not eating, hanging at the surface or lying on the bottom, odd swimming.

What it isn't

Ammonia or nitrite burns: reddened gills and fins can come straight from bad water, without infection. Test first; see ammonia and nitrite.
Natural red colour: some fish have red fins or patches. Look for change, not just colour.
Fin rot: ragged, eroding fins. It can lead to septicemia if it spreads.
Injury: a single red mark after a fight or a scrape on decor. Watch it; it should fade in a few days.

Not the same as VHS

Viral hemorrhagic septicemia (VHS) is a different disease caused by a virus, mostly in wild and farmed fish such as trout, salmon, perch, walleye and pike, including in the Great Lakes. It is a notifiable disease internationally and is reportable to animal health authorities in the US and Canada. It isn't a normal aquarium problem. If you keep or catch native coldwater fish and see mass deaths with bleeding, contact your local fisheries or animal health agency rather than treating at home.

When it's a problem

Early: faint red streaks in one fish's fins, fish still eating. Act today; this is the best time to turn it around.
Serious: blotches on the body, sores, not eating, or more than one fish affected.
Emergency: swollen belly or pine-coning, bulging eyes, lying on the bottom, or fish dying one after another. Act within the hour.

What to do right now

Test the water: ammonia, nitrite, nitrate, pH and temperature. Poor water is the most common trigger.
Do a 30 to 50% water change with dechlorinated, temperature-matched water. Repeat daily if ammonia or nitrite is above zero.
Move the sick fish to a hospital tank with a heater, air stone and a mature filter sponge if you have one. This protects tank mates and makes treatment easier. See quarantine and hospital tanks.
Remove dead fish immediately. Dead fish release huge numbers of bacteria. See dead fish.
Add aeration and keep the temperature steady.
Wear gloves and wash your hands well. The bacteria involved can also infect people through cuts.
Take clear photos and write down your test results, then contact a vet who treats fish.

What to do next

Antibiotics through a vet

Septicemia is an infection inside the body, so it usually needs an antibiotic that gets into the bloodstream. That's a job for a vet:

Many countries, including Canada, the UK and the EU, require a vet prescription for antibiotics for fish, and US rules have tightened in recent years too. Most over-the-counter "antibacterial" fish products are not true antibiotics, or don't reach the blood well.
Aeromonas often resists one or more common antibiotics. Vets recommend a culture and sensitivity test where possible, so the chosen drug actually works.
Depending on the fish, a vet may prescribe medicated food (if the fish is eating), injections (for larger fish like koi), or a medicated bath.
Follow the vet's dose and timing exactly and finish the full course, even if the fish looks better. Stopping early breeds resistant bacteria.
Don't use antibiotics "just in case" or for viral or parasite problems. They don't help and make resistance worse.

While treating

Remove carbon from the filter if you treat in the water.
Watch ammonia closely. Some antibiotics in the water can harm filter bacteria, so test daily and change water as needed.
Scaleless fish, catfish and loaches can react badly to some medications; tell your vet what species you keep.
Shrimp, snails and plants can be harmed by some water treatments. Another reason to treat in a hospital tank.
Salt at a low level can help some fish cope with damaged skin, if the species tolerates it. See aquarium salt. It doesn't cure septicemia.
Feed lightly with high-quality food if the fish eats.

Fix the cause in the main tank

Fish health references agree: these infections usually follow a predisposing problem, and losses usually stop once that problem is corrected. Look for:

Ammonia or nitrite from overfeeding, overcrowding or a filter that's been cleaned too hard.
Low oxygen, especially in warm water.
Sudden temperature changes or a chill.
Injury from handling, nets, sharp decor or fighting. See aggression.
Another disease, such as parasites or columnaris, that damaged the skin first.
Rotting food or a dead fish hidden in the tank.

When to consider euthanasia

Some fish are too sick to recover, especially with heavy bleeding, a badly swollen belly, raised scales or lying on their side for days. If a fish isn't eating, is getting worse despite treatment, or is clearly suffering, a humane end is kind. A vet can do it, or advise you on an accepted method. Don't flush a living fish or put it in the freezer. See dead fish for disposal.

Products that help

Vet-prescribed antibiotic (medicated food, bath or injection): the actual treatment for internal infection.
Hospital tank with heater, air stone and sponge filter: for isolating and treating the sick fish.
Liquid test kit: to find and fix the water problem behind it.
Dechlorinator that binds ammonia: short-term protection during a spike.
Gloves: for handling sick fish and tank water.
Pure non-iodised salt: supportive care for salt-tolerant fish only.

How to prevent it

Keep ammonia and nitrite at zero and change water weekly.
Don't overcrowd or overfeed.
Handle fish gently, use the right size net, and avoid sharp decor.
Keep temperature stable.
Quarantine new fish for 3 to 4 weeks.
Remove dead fish and uneaten food quickly.
Treat parasites and skin damage early before bacteria move in.

Why it happened

The bacteria that cause septicemia live in almost every aquarium and pond, on surfaces and even inside healthy fish. They break through when a fish's defences are down: poor water, injury, chilling, crowding, spawning stress or another disease. Some strains are very aggressive, but most cases start with stress.

When to get help

Call a vet who treats fish as soon as you see red streaks plus any other sign of illness, if more than one fish is affected, or if a fish has stopped eating. Many vets can help remotely with photos and water test results. Contact your local animal health or fisheries authority if you suspect VHS or see mass deaths in wild or pond fish.

In the game

Multiple Tank Syndrome doesn't have a septicemia card yet.