Bacterial infections in aquarium fish: fin rot, columnaris and dropsy
Ragged fins, white patches on the mouth or a swollen belly: bacterial infections almost always start with poor water or stress. How to recognise and tackle them.
Bacteria are present in every aquarium. A healthy fish isn’t bothered by them, but with stress, injuries or poor water they can strike. So the most important medicine is almost always: better water.
Fin rot
Symptoms: fins that become ragged, develop white or red edges and keep getting shorter. In serious cases the infection also attacks the body.
Causes: poor water (high nitrate, a dirty substrate), fin-nipping tankmates or injuries.
Approach: change water, clean the substrate, tackle the cause. Fins often grow back by themselves once the water is right. In serious cases an antibacterial treatment may be needed.
Columnaris
Symptoms: white or grey patches on the mouth, the back (‘saddleback’) or the gills that look like fungus. That’s why it is also called ‘cotton mouth’, but it is caused by the bacterium Flavobacterium columnare.
Important: columnaris can develop extremely quickly, especially at higher temperatures. Act fast, and don’t raise the temperature as you would for white spot.
Approach: isolate sick fish, improve the water and treat with a suitable product. Get advice from a specialist shop or vet.
Dropsy
Symptoms: a swollen body with scales standing on end (the ‘pine cone’ look), sometimes bulging eyes.
What is it? Not a disease in itself, but a sign that the organs, often the kidneys, are no longer working properly, usually due to a bacterial infection. The chance of recovery is small. Isolate the fish and check your water and your other fish.
Ulcers and red patches
Open sores or red patches on the skin often point to an infection with bacteria such as Aeromonas. Usually an injury or poor water quality comes first.
Fish TB (mycobacteriosis)
Slow weight loss, a bent spine, bulging eyes and sores that don’t heal, often in several fish one after another. There is no reliable treatment. Note: this bacterium can cause skin infections in people. Wear gloves if you have cuts on your hands.
Antibiotics
In the European Union, antibiotics for animals are only available on prescription from a vet. Never use them casually: they also kill the useful bacteria in your filter, and misuse makes bacteria resistant. Always treat with antibiotics in a separate tank.
Yawar deliberately gives no dosages: always follow the product’s instructions and, when in doubt, ask a vet experienced with fish. Remove activated carbon from the filter during treatment, and note that many treatments harm shrimp and snails.
Prevention
- Change water weekly and vacuum the substrate.
- Don’t overfeed: leftover food spoils the water.
- Not too many fish, and no fin nippers with long-finned species.
- Quarantine for new fish.
- A stable temperature and a properly working filter.