Why fish get bacterial infections
What You Need To Know:
Harmful bacteria is likely ever-present in most aquariums. However, a fish's strong natural immune system + excellent water quality will usually keep it at bay.
Infections can be caused by either Gram‐positive or Gram-negative bacteria. Gram-negative infections are typically more virulent, and more common
in marine fish.
Bacterial infections are sometimes "secondary" to a preexisting parasitic or worm infestation.
Poor water quality, open wounds and nutritional deficiency (which lowers the natural immune system) are all possible contributing factors of infection.
The best treatment for a bacterial infection is a broad-spectrum antibiotic: Antibiotics
Most aquariums contain both good (e.g. Nitrifiers) and harmful bacteria. The latter invariably gets introduced via corals & inverts even if you quarantine your fish. This "bad" bacteria is usually kept in check by a healthy fish's natural immune system. Or if a fish becomes sick and shows signs of an infection, sometimes
the immune system is still able to fight it off without the aid of antibiotics
. So, if you stop and think about it, these bacterial infections with fish are akin to our own never-ending battle with germs, viruses, and of course, infections.
Many factors make a fish more susceptible to infection. First, a cut or open wound is usually required as an "entry point" for harmful bacteria to attack. Even tiny "bite marks" left by feeding parasite trophonts or monogenean flatworms can get infected. Poor water quality (i.e. dirty water) can be a contributing factor which increases the odds of a bacterial infection occurring. Also, anything that lowers a fish's overall immune system makes infection more likely. Stress (e.g. fish fighting), malnourishment, or if the fish is battling an ongoing parasitic or worm infestation makes a "secondary" bacterial infection more possible. For example, back when I practiced "Ich management
" it would seem periodically that a fish would develop a cloudy eye or some suspicious spot on it's body. Now that I quarantine (QT
) and prophylactically treat all of my fish, I almost never see anything like that once the fish enters my display tank (DT
). Disease-free fish are healthier and thus more capable of overcoming potential infections.
Gram positive vs. gram negative:
Unfortunately, the majority of bacterial infections in marine fish are caused by gram-negative bacteria. These are typically more virulent than infections caused by gram-positive bacteria. When a fish does have a gram-positive infection, symptoms may be so mild that they are unnoticeable. The only way to differentiate between the two is to take a skin scrape of the affected area and gram stain it (instructions here
, and also here
, and a video tutorial of gram staining
). Gram-positive bacteria stain blue/purple, and gram-negative bacteria stain pink/red.
Visible Physical Symptoms of a Bacterial Infection
- There are photos of fish with bacterial infections above. Keep a close eye out for any of the following:
- Any "redness" or open sores/wounds on a fish should be viewed with suspicion.
- A white film or "fungus" looking growth can denote a bacterial infection.
- Frayed fins / fin & tail rot.
- Cloudy eyes.
- Bloating can mean that the fish has an internal bacterial infection.
However, it is important to note that the symptoms listed above can sometimes mean something different. For example, a white "cauliflower-like" growth on the fins & spines is most likely just Lymphocystis
; a benign virus found in many fish. Redness around the gills is a symptom of ammonia burn, while open red sores on the fish could be Uronema
. Although rare, sometimes "true" fungus can afflict marine fish. So, doing some investigating and not just lumping everything into one category is important. Whether or not a bacterial infection is contagious is highly dependent upon which species of bacteria is attacking your fish, and also the overall conditioning of your other fish. So, there is no easy answer when dealing with this problem.
- First off, there are many steps you can take to prevent a bacterial infection from occurring in the first place. Some of these include:
When to medicate:
- Maintaining a good environment (e.g. clean water, proper size, stress free) for your fish to live in.
- Separating two quarreling fish before cuts/wounds get too serious.
- Utilizing proper nutrition (e.g. live foods, nori, frozen seafood), and soaking fish food with vitamin supplements, probiotics & Beta-glucan. These will help boost your fish’s natural immune system.
- Utilizing a Quarantine Tank - to prevent parasites and other pathogens from ever entering your DT. This will alleviate the possibility of a "secondary" bacterial infection popping up while the fish's immune system is already compromised from battling parasites/worms.
- Running a UV sterilizer, Ozone or some other disease management tool may help in certain situations. These can help lower the overall number of harmful bacteria found in the water column.
Sometimes all the vitamins, proper nutrition and clean water are just not enough. Sometimes a fish's immune system needs a helping hand (like our own). When to use antibiotics
is a judgment call. If the sick fish is in a reef, you can first try lacing the fish food with antibiotics (recipe here - scroll down)
. It's very important
to use Seachem Focus
or some other binding agent (e.g. agar, unflavored gelatin
) to protect your corals & inverts. As a general rule, I only pull a sick fish & treat (dose) with antibiotics in a QT if: (a)
The fish looks really bad or (b)
It is a newly acquired fish showing signs of infection. The latter is an easy call for me as I quarantine
all new fish anyway. Here is a list of antibiotic medications that you can use: Antibiotics
More info about specific antibiotics: Aquarium Medications Part 2 | Antibiotic & Antimicrobial Treatments
There are also certain baths that you can administer to help treat a fish with a bacterial infection:
More information below:
Use of Antibiotics in Ornamental Fish Aquaculture
Bacterial Diseases of Fish
- Ciprofloxacin (Best administered via 1-2 hour bath treatment. Dosage is high: 250mg per gal. Repeat every 24 hours for 7 days. Methylene Blue can be added to increase efficacy.)
- Nitrofuracin Green Powder (100mg per gallon if doing just a 30 minute bath)
- Ruby Reef Rally bath for 90 minutes
- Methylene Blue bath for 30 minutes
- Also any other antibiotic can be used in a bath treatment for 30 minutes at double dosage.