Red pest is a bacterial infection in freshwater aquarium fish that causes red streaks, blotches, and sores on the skin, fins, and body. The disease is a form of hemorrhagic septicemia, meaning bacteria have entered the bloodstream and are causing internal bleeding visible through the fish’s scales. It most commonly affects goldfish, koi, and other coldwater species, though tropical fish can develop it too. Left untreated, red pest progresses quickly and is often fatal.
What Red Pest Looks Like
The hallmark sign is red streaking or blotching, typically appearing first on the fins, tail, and belly. These marks aren’t surface injuries. They’re blood leaking from tiny vessels under the skin as bacteria damage tissue from the inside. In early stages, you might notice faint pink lines running through the fins, especially the tail fin. As the infection advances, those lines deepen into vivid red streaks, and broader patches of redness spread across the body.
Behavioral changes usually accompany the visible symptoms. Infected fish often become lethargic, stop eating, clamp their fins tight against their body, and may isolate themselves near the bottom of the tank or hover at the surface. In advanced cases, scales may lift away from the body (a condition called dropsy), open ulcers can form, and fins begin to erode. Once ulcers or dropsy appear, the infection has become systemic and is much harder to reverse.
What Causes It
Red pest is caused by gram-negative bacteria, most commonly Aeromonas hydrophila and related species. These bacteria are naturally present in most aquarium water at low levels. They only become a problem when a fish’s immune system is compromised, which is why red pest is considered an opportunistic infection rather than something introduced from outside.
The triggers are almost always environmental. Poor water quality is the biggest culprit: elevated ammonia, nitrite, or nitrate levels weaken a fish’s natural defenses and give bacteria an opening. Overcrowding, sudden temperature swings, aggressive tankmates, inadequate filtration, and the stress of transport or a new environment all raise the risk. A fish that’s been recently shipped, moved between tanks, or kept in degraded water conditions is significantly more vulnerable.
Red Pest vs. Ammonia Burns
Red blotches on a fish don’t always mean bacterial infection. Ammonia burns can produce similar-looking redness, particularly on the gills, fins, and belly. The key difference is context. If your ammonia or nitrite readings are elevated and the redness appeared suddenly across multiple fish, poor water quality alone may be the cause, and a large water change can resolve it. If your water parameters test clean but a single fish is developing spreading red streaks alongside lethargy and appetite loss, bacterial infection is the more likely explanation. In practice, ammonia stress and bacterial infection often overlap: the poor water weakens the fish, and bacteria take hold.
How to Treat Red Pest
Red pest requires antibiotic treatment. Salt baths and water changes alone won’t clear a systemic bacterial infection, though improving water quality is an essential first step. The bacteria are in the bloodstream, so the medication needs to reach the fish internally, not just contact its skin.
Experienced fishkeepers often use a three-pronged approach simultaneously: antibiotics mixed into food, medicated baths, and medication added to the aquarium water. Each serves a different purpose.
- Antibiotics in food are the most effective route because the medication is absorbed through the gut and enters the bloodstream directly, reaching therapeutic levels where the bacteria actually live. Broad-spectrum antibiotics work best here.
- Medicated baths deliver a concentrated short-term dose. For furan-based medications, use five times the package-recommended dose; for sulfa-based drugs, use ten times. Place the fish in the bath for exactly one hour, then return it to the aquarium. Repeat once every 24 hours. Furans and sulfa drugs work similarly, though furans are somewhat more toxic to fish, so sulfa-based options are gentler.
- Medication in the tank water at normal package-recommended levels won’t bring drug concentrations high enough inside the fish to kill bacteria, but it does prevent the infection from spreading to tankmates.
Running all three treatments at once gives the best chance of saving a fish with active hemorrhagic septicemia. If the fish is still eating, that’s a good sign, since getting antibiotics into the food is the single most effective step. A fish that has stopped eating entirely is harder to treat because you’re limited to baths and water-column medication.
Why Quarantine Matters
New fish are the most common way bacterial problems enter an established tank. A fish that looks healthy at the store may be carrying bacteria at subclinical levels, only to break with disease days later once shipping stress catches up. Quarantining every new fish for at least four weeks before adding it to your main tank is the single most effective prevention strategy.
A quarantine tank doesn’t need to be elaborate. Most people run them bare-bottom with no plants or decorations, which makes observation easier and cleanup simpler. The tank does need a cycled filter, ideally one that’s been running for at least six weeks to build up a mature bacterial colony. Keep the quarantine setup in a quiet room, and consider draping blankets over the tank for the first day or two to let new arrivals de-stress. Minimize noise, foot traffic, and disturbances.
During quarantine, many fishkeepers proactively feed a broad-spectrum antibiotic mixed into the food for ten days. This clears subclinical bacterial loads before the fish ever enters the display tank. Oral antibiotics are the only reliable route here; adding them to the water column alone won’t reach effective levels inside the fish.
Keep all quarantine equipment, including nets, buckets, and siphons, completely separate from your main tank gear. Treat everything that touches the quarantine water as potentially contaminated. Once a fish has gone four weeks with no signs of illness (or four weeks after resolving a problem), it’s generally safe to transfer it to your main system.
Preventing Outbreaks in an Established Tank
Since the bacteria behind red pest are opportunistic, prevention comes down to keeping fish healthy enough to resist them. That means stable, clean water above all else. Regular water changes, adequate filtration for your stocking level, and consistent temperature all reduce the bacterial load in the water while keeping your fish’s immune defenses strong.
Overcrowding is a common trigger people overlook. More fish means more waste, higher bacterial counts in the water, and more social stress, all of which compound. If red pest keeps recurring in your tank despite treatment, the root cause is almost certainly environmental. Treating the infection without fixing the underlying water quality or stocking issue means the disease will return.
Feeding a high-quality diet also plays a role. Well-nourished fish mount stronger immune responses. Avoid overfeeding, which degrades water quality, and remove uneaten food promptly. A tank with good filtration, stable parameters, moderate stocking, and a proper quarantine routine for newcomers will rarely see red pest at all.