The best defense against fungus is a clean tank with lots of room for each fish. If your fish do get fungus, treat it immediately. Salt is the most widely recommended treatment, 1 tablespoon per gallon of water. It can be aquarium salt from the fish store, rock salt from the hardware store, or sea salt from the grocery store, as long as it isn't iodized. Other treatments include fish drugs malachite green, methylene blue, potassium permanganate, and fulvinex. Use these according to package instructions. They all will kill plants and snails, so if this is a concern, you should treat your sick fish in a separate tank. Some fish keepers report success with melaluca (tea tree oil) based fish medications, which do not kill plants or snails.
Fish & Aquarium Supplies: Bacteria & Fungus Medications
However do not confuse Columnaris with the spores of Saprolegnia the cotton wool ball with hair like growth structures seen on damaged fish. Without a microscope, Columnaris can look like Saprolegnia and is often treated as fungus, sometimes with poor results.
The best way to tell with the naked eye (using a magnifying glass) is the hair like growth structures of Saprolegnia/Fungus.
WITH a microscope, scrapings from a columnaris lesion then placed under a microscope will reveal long, thin, rod shaped motile bacteria. The bacterial clumps form microscopic columns or dome shaped masses, hence the name columnaris.
Columnaris has similarities to Aeromonas bacterial infections in that both are opportunistic (both generally present in some location of an aquarium), however since Aeromonas is a facultative bacterium & often anaerobic, it is more prevalent in a tank with high amounts of pollution, DOC and especially poor circulation.
While Columnaris being aerobic (preferring high oxygen environments) can occur in tanks with good filtration/circulation, however it is common in a tank with poor Redox/mineralization, overcrowding, high temperatures, and stress (such as an aggressive fish tank where many inhabitants are constantly bullying others).
As well weak genetics from in-bred fish (as with many Guppies and Bettas) or fish such as "feeder" or "carnival" goldfish that are often permanently damaged for early life crowding are also often at risk for Columnaris infections.
Unfortunately the above point is missed by many who often cite dirty, stagnant, or otherwise poor water conditions as cause of Columnaris, but since Columnaris is , it simply thrive in poor water conditions that are low in oxygen as can Aeromonas or Saprolegnia.
This is NOT to say that sudden shifts in parameters such as spikes in ammonia (assuming an aerobic environment), can will not trigger Columnaris, as this too is a stressor that can allow for an opportunistic Columnaris infection.
HOWEVER, as I have noted many times in my years in aquarium maintenance and "sick fish" service calls; dirty, foul, under filtered, low oxygen environments, RARELY trigger Columnaris infections.
Adding aeration without correcting mineralization problems based on false assumptions about Columnaris while ignoring ESSENTIAL mineral ions (Calcium in particular, but sodium chloride too) will only further force an out of balance Redox and not help with a cure of a true Columnaris infection!!
In lieu of often unavailable scientific microscopic identification, the above point is often noteworthy in identifying aerobic Columnaris from often anaerobic Aeromonas or the Mold Saprolegnia (generally referred to as "Fish Fungus").
Please read further for a better understanding of this bacterial disease as well as treatment and prevention (there is also a section about as well)
How Fast Does Body Fungus in Aquariums Clear Up? - Pets
Fungus is as the name tells us caused by fungi (Saprolegnia) and will only attack weakened fish that has already been infected by another disease. Fungus will never attack an otherwise strong and healthy fish and the best way of avoiding this disease it therefore to keep your aquarium clean and your fish healthy and well fed.
Fungus will eventually kill infected fish when it has spread through the fish even if this often takes quite a while to happen. The symptoms of fungus are gray or white growth on the fins and skin of the fish. The gray or white areas grow larger and more cotton like as the disease progresses.
Fungus is fortunately rather easy to treat and there are several different ways to treat it. One is to add methylene blue to the water. Methylene blue are most commonly used to prevent fungi on fish eggs but can be used on adult fish as well even if it isn't necessarily the best option for adult fish. Personally I recommend using methylene blue for treating and prevent fungi on eggs and fry. On adult fish I instead recommend using a phenoxethol solution or commercial antifungal medicine designed to treat fungus in fish. If you choose to use phenoxethol solution you should add 10ml of 1% phenoxethol solution per litre aquarium water. The treatment can be repeated after a few days if needed but should not be done more than twice since repeated use can have damaging side effects. Severely infected fish can be caught and swabbed with a povidone, iodine or mercurochrome soaked cloth to increase the effectiveness of the treatment. Make sure to increase the water quality and fish health in your aquarium once the fungus has been cured to avoid the problem in the future.
HOW TO TREAT FUNGUS ON FISH! - YouTube