Ulcers in Freshwater Fish

Red-bellied pacu freshwater aquarium fish
kazakovmaksim / Getty Images

Ulcers in freshwater fish are a common problem, but they are usually not the only problem you may have at the time. Ulcers most commonly occur when a fish's defenses are weakened by a primary stressor, allowing bacteria to enter and disrupt the skin. Severe ulcers may require veterinary care, but basic ones are easily treated by checking a few key items.

What Are "Ulcers?"

Ulcers are defined as a sore on the skin resulting in tissue necrosis. They can have many inciting factors that start the injury. Severe ulcers may lead to exposure of underlying musculature. Ulcers are more common over muscle tissues since they carry more blood flow.

Due to the nature of fish systems, bacteria are always present on fish skin. Most of the time, a fish's immune system will keep the bacterial invaders from penetrating the outer skin layers. Fish skin mucus is full of immune factors, which is why it is important to keep it on the fish. Water conditioners promoting "improved slime coat" are ineffective, but do promote wound healing.

If a fish's immune system cannot keep up with the bacterial onslaught or you encounter a particularly nasty bacteria strain, your fish are likely to develop ulcers. Although they may start on the surface, once a bacteria has breached the outer wall, your fish may be susceptible to a full systemic infection or sepsis.

Signs of Ulcers in Freshwater Fish

  • Elevated scales
  • Increased veining (erythema)
  • Scale loss
  • Red spots (varying sizes)
  • Skin erosion
  • Exposure of underlying muscles
  • Loss of fin structure
  • Lethargy
  • Decreased appetite
  • Death
Koi with side ulcer
The Spruce / Jessie Sanders

Causes of Ulcers in Freshwater Fish

Primary infections in fish of ulcerating bacteria are rare. Often, a common bacteria species will take advantage of a drop in your fishes' immune defenses. An acute stress reaction is beneficial for fish, and is associated with the "fight or flight" response. If a stressor cannot be escaped or mitigated, this results in a chronic stressor. Chronic stress results in decreased immune function, decreased fecundity and decreased body condition. These are some of most common stressors for freshwater aquarium fish, but not all:

Poor Water Quality

The water a fish swims in is directly correlated to their overall health. Not maintaining your system and ignoring water quality will result in many secondary diseases, ulcers included.


Freshwater fish parasites can stress fish out and create entrances for bacteria when they flash. Flashing is a behavior in which fish use their substrate and any decor to scratch themselves. This behavior may look like a "twitchy dance" or "uncoordinated swimming." Parasites will often enter a system when new fish are added without quarantine.

Keep in mind that your system temperature will dictate how fast parasites complete their life cycle. This is critical of outdoor ponds with varying seasonal temperatures.

Poor Nutrition

In order to give your fish the best nutrition, don't just reach for whatever is most expensive. Do your research so you know what diet is best for your fish. Poor or inadequate nutrition will decrease your fishes' immune function.

Don't keep any food for more than 6 months; after this point, it has started to lose too much of its vitamin content. Keep your food inside in a sealed, light-proof container in a pantry or cupboard.


Fish can become aggressive over food, territory and reproductive opportunities. Aggressive behavior can include charging, nipping, ramming and chasing. The stress of anticipating an attack can be enough to stress out more docile species. Open wounds from attacks in addition to stress can create an ideal ulcer environment.

Treatment of Ulcers in Freshwater Fish

Once a primary stressor has been identified and removed, your fish will most likely heal without any additional intervention.

Severe ulcers will require veterinary treatment, often with antibiotic therapy. Your veterinarian may recommend a culture and sensitivity test to identify the primary bacteria of concern and a list of antibiotics it is most susceptible to.

Do not attempt to treat your fishes' ulcers with over-the-counter antibiotics. These illegal products are not checked for potency and efficacy. Water-based treatments will also wipe out your biologic filter and cause additional stress.

How to Prevent Ulcers in Freshwater Fish

In order to prevent ulcers in fish, it is critical to mitigate stress in your system. It is exceptionally rare that a deadly bacteria will get into your system if you are...

  • Maintaining your water quality,
  • performing your regular maintenance,
  • feeding an appropriate and fresh diet,
  • using proper biosecurity protocols,
  • and completing proper quarantine.
Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Shivappa, Raghunath B. et al. Laboratory Evaluation Of Different Formulations Of Stress Coat® For Slime Production In Goldfish (Carassius Auratus) And Koi (Cyprinus Carpio)Peerj, vol 5, 2017, p. e3759, doi:10.7717/peerj.3759

  2. Harms, Craig A. Therapeutics for Fish. North American Veterinary Conference.