Practical Fishkeeping Diploma - Week 4, Day 3

Here's the third of a five-part series of lessons to guide new aquarists through the basics of fishkeeping.

Week 4, Day 3: Bacterial diseases

Bacterial diseases

Bacteria are single celled organisms that reproduce through self-division to form colonies. Aquaria rely on certain benign bacteria species to convert down aquarium wastes inside the filter, but other bacteria can be pathogenic and cause disease.

Bacterial infections only take a few common forms in aquaria, and most are directly associated with acute or chronic stress.

Ulcers are caused by Aeromonas, Pseudomonas, Vibrio, Mycobacterium and Flavobacterium genera of bacteria, present in aquarium water at all times. These are opportunistic and infect fish that are already suffering from (often chronic) stress and/or physical injuries. 

Signs: depressed, rounded wounds on the fish, usually red and with a white edge. Extreme cases may expose a fish’s internal organs.

Treatment: off the shelf broad range antibacterial medication (usually formalin based). 3g/l of salt will help. Extreme cases may require prescribed antibiotics added via food. Some antibacterial treatments can be applied direct to the fish and sutured with a ‘wound sealer’. Tea tree oil-based medications (Melafix).

Note: the presence of ulcers causes osmotic stress in fish. Open wounds allow essential minerals to leave (or excessively enter) a fish’s body.

Fin rot is caused by the same bacteria as ulcers that get into the fish through different entry points, specifically if there has been any physical injury to the fins.

Signs: erosion of tissue between one or more fin rays, often with a white edge. Fin rays and spines often left intact, giving the fins a ‘comb’ look.

Treatment: off the shelf medicines containing phenoxyethanol. 3g/l salt will help. Tea tree oil-based medications (Melafix). If the bacteria reach the body, the disease escalates rapidly, and treatment as for ulcers may be needed.

Cotton mouth or columnaris is caused by Flavobacterium columnare bacteria. Outbreaks are linked directly to poor aquarium husbandry and sources of chronic and acute stress as we looked at earlier in the module. Poor water quality is a particular culprit, with elevated levels of ammonia, nitrite or nitrate being common precursors to the disease.

Signs: white or grey lips, cottony tufts around and on the mouth, shimmying (where the fish twitches and shakes), reluctance to feed, red patches on the body, grey patches over the head and gills. Eventually the mouth may erode away altogether.

Treatment: formaldehyde or phenoxyethanol based bacterial medications may help, along with 3g/l salt. Severe infections require antibiotics.

Fish TB or Wasting disease is caused by Mycobacterium marinum and Mycobacterium fortuitum carried by infected fish. Chronic stress from poor husbandry and poor water quality will trigger outbreaks. Cannibalism and fish eating infected faeces transmits the disease.

Signs: emaciation, pale colours and general poor health. External diagnosis difficult, as internal organs are infected. Dissection of dead fish will expose white nodules in the body.

Treatment: strong antibiotics may have some effect. Salt and off-the-shelf medicines generally ineffective.

Fish TB is a zoonotic illness that can rarely transfer to humans through cuts in the skin.

Click here for next lesson: Parasitic diseases

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Diagnosing your fish and treating with API Fish Remedies

API Fish Disease Chart

Guide to fish healthcare