Fish Ulcers

One of the most common owner concerns we get calls about are “ulcers” on their fish? What is an “ulcer,” how do they come about and how do you fix them?

What is an “ulcer?”

Most commonly, fish get skin ulcers. This is an open wound in the skin of a fish that may expose the underlying musculature. In fish, it can be accompanied by scale loss, local growth of fungus and may appear suddenly or grow slowly over time. They can appear anywhere on the body of the fish.

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How do ulcers occur?

Since fish, essentially, live in a toilet, they are constantly fending off external attacks from parasites, bacteria and fungus. Their excellent immune system works hard to keep these invaders from entering a fish’s body. When the immune system slows down or stops working, from fish stressors (#1 = water quality), the invading pathogens get to work on the fish’s most external defense network: the skin.

Once the immune defenses have been breached, fungi and bacteria can get set into a fish’s skin and set up a happy home. Parasites are usually secondary, with a fish’s behavioral flashing opening up sites for bacteria and fungi to enter.

How do you fix fish ulcers?

It is rare for an ulcer to occur spontaneously, without any primary triggers. Most ulcerations are caused secondary to some other stressor (#1 = water quality). Without identifying the primary cause, no matter what you use to treat, the ulcer will return.

Once the primary stressor has been identified and corrected, most mild ulcers will heal on their own. More severe ulcers, especially those penetrating in the muscles, may require antibiotic therapy, prescribed by your aquatic veterinarian. Prescription medicated feeds may be required for a tankwide or pondwide outbreak.

There are some very tough bacterial bugs in the fish world, most commonly Aeromonas and Pseudomonas spp in koi and goldfish, that are capable of causing primary ulcerations. These are VERY RARE and are usually in fish that receive a lot of antibiotic treatment, most commonly over-the-counter drugs for diseases they do not have. They are diagnosed by a culture and sensitivity test, performed by an aquatic diagnostic laboratory. To prevent these highly resistant organisms from getting in your tank, only use antibiotic medications when prescribed by a veterinary professional.

How do I prevent fish ulcers?

Mitigating stress in your fish’s environment is key. The best preventative medicine in fish is good water quality and good diet. If you suspect your fish has an ulcer that is not improving despite checking the above factors, contact your local aquatic veterinarian immediately.

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