Practical Fishkeeping Diploma - Week 4, Day 2

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

Week 4, Day 2: Fungal and viral disease

Infectious diseases are those caused by pathogens and parasites, and those are what we shall look at in the next three days of the diploma. Infectious diseases can transfer from one fish to another. Zoonotic diseases can even transfer to humans or other animals and need extra care when encountered.

With these diseases, fish can be subject to a primary or a secondary infection. Primary infections arise when healthy fish succumbs to a pathogen or parasite invasion.

Secondary infections arise when a fish that is already damaged (through physical injury, stress or when suffering from a primary infection) is attacked by an opportunistic pathogen that otherwise wouldn’t be able to affect the fish. Typical examples include bacteria entering the body through cuts, as in the case of finrot.

Fungal diseases

Fungi are ever present in aquaria as spores, and are opportunistic. The majority of fungal infections in fish are external, although there are rare instances of internal infections.

Cotton wool disease is caused by Achyla and Saprolegnia fungi. Poor water quality, poor hygiene, chronic stress and injury are all factors in an outbreak. It often follows in the wake of diseases such as ulcers or fin rot.

Signs: obvious patches of white fluff, usually where the fish has been injured. Patches may turn green or brown over time. Spreads from damaged flesh to healthy flesh, degrading it by sending hyphae down to digest healthy tissue.

Treatment: rectify all predisposing factors, dose aquarium with off the shelf, phenoxyethanol based medicine. 3g to 5g salt per litre of tankwater during an outbreak will inhibit the spread of fungus. Bay tree oil-based products such as Pimafix.

Viral diseases

Relatively rare in aquaria, viral infections pose a particular problem as they cannot be treated. Viruses ‘hijack’ living cells to create more viruses. Sometimes, invaded cells become so large that the infection becomes visible as lumps on the body. Internal virus infections are hard to diagnose without specialist equipment. As well as the illnesses below, viruses may also be implicated in tumours and dropsy outbreaks.

Lymphocystis, or Cauliflower disease is caused by Lymphocystis or similar viruses. Chronic stress appears to be an underlying cause, though the condition is often synonymous with fish that have been injected with artificial dyes. The virus may spread via injuries, though cannibalism of dead fish may be a factor.

Signs: small pink or white-grey lumps that can grow large and ‘cauliflower’ like. Infected fish may appear unbothered by the growths. In extreme cases, they may lose condition and refuse to feed, eventually dying.

Treatment: not possible.

Dwarf gourami iridovirus is caused by a yet to be identified iridovirus, the disease attacks Dwarf gouramis, Trichogaster lalius, as well as Betta splendens, and possibly Angelfish and Ram cichlids, amongst others.

Signs: (in Dwarf gouramis) emaciation, loss of appetite, reduced colours, belly bloating, open sores and death.

Treatment: not possible and the underlying factors not clearly understood.

Click here for next lesson: Bacterial 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