These are ubiquitous opportunistic bacterial invaders in every pond! At one time they were the only concern hobbyists had when buying fish, and they were the number one cause of fish death in spring and summer in the United States. They have been replaced by KHV but as they exist in every pond, we need to pay attention to what will cause an outbreak, and prevent it.

The first precaution is construction. A pond with sharp edges is an infection waiting to happen. Rocks at the edge of the pond will snag fish and create an opening in the epidermal protective slime layer, offering an invitation to any primary or secondary infection. As it is easier to prevent infection than cure it, it is recommended to check for any sharp objects that have the potential for fish to scrape themselves.

The second is the water quality. A clean pond will contain more good-guy bacteria and less of the bad guys. Parasites are the second way these bacteria will enter the endodermis of the fish. Parasites proliferate in dirty water and debris on the bottom. Filters that have not been cleaned will harbor the same parasite populations as the filth on the bottom of the pond.

And third, a healthy fish is able to resist many infections! Things which will cause stress for the fish, thus reducing their resistance, are changes in environment, dirty water, too high or too low pH, ammonia, nitrites, overcrowding, over feeding, inadequate filtration or water movement, handling of fish, poor nutrition, excessively high or low temperatures, toxins in the water, predators, etcetera.

What does Aeromonas or Pseudomonas look like? It will be seen first as a reddening of the skin which develops into an ulcer. These two bacteria strains will attack damaged tissue, killing tissue beside it as it multiplies. The ulcer will grow in size. Scales can easily be removed from the sore. Fungus and algae may attach to the dead tissue. The fish is literally being eaten away. Without treatment, the fish will die.

Treatment consists of intraperitoneal injections by a trained person, as a wrong move can also kill the fish. The sore can be swabbed with iodine or other topical medicine. The fish should remain in quarantine for the duration of treatment to reduce the stress of catching it daily. The pond should be examined for the cause of the infection, treated if parasites are found (by a scraping viewed under a microscope). The rest of the fish should be watched carefully for signs of infection. This is not contagious as once believed, however if there is something in the pond that opened this fish to infection, it will open other fish too.