Diseases are generally not a problem in aquariums which have over-filtered, crystal clear, bacteria-free water.
The Key to Good Fish Health is Clean, Clear, Bacteria Free Water
Note this does not mean water which is changed frequently. The idea that water changes create good health is a myth. The idea that water which has “good water parameters” will give good health is also a myth.
We go into this in much greater detail in this article:
Bacteria in the Water Column
The fish’s immune system is constantly fighting all the bacteria that the gills are exposed to regardless of the type of bacteria. So a lower bacterial count means more immune system resources available for fighting off pathogens. A high bacteria concentration will compromise the immune systems of the fish and can cause any number of diseases and pathogen breakouts. The easiest way to determine the amount of bacteria in the water is by water clarity. In general, the clearer the water, the fewer bacteria. All owners of aquariums should strive for crystal-clear water.
Note that “bacteria-free” is relative. It is all on a logarithmic scale. A milliliter (or cubic centimeter) of dull, “normal” aquarium water will have roughly ten million or 10⁷ bacteria. A milliliter of cloudy water can easily have ten billion or 10¹⁰ bacteria. And a milliliter of “crystal clear” water can have as little as 100,000, or 10⁵ bacteria in it. This means that cloudy water has 100,000 times more bacteria in it than does crystal clear water. This gives one an idea as to why reducing the bacterial count can be so effective in improving the immune systems of the fish.
If the bacteria count in the water column is low the fish can put all their immune system resource towards keeping pathogens at bay. So fish in bacteria-free water is far more healthy than fish in bacteria-laden water. This is simply the best disease preventative there is.
Note that, contrary to popular myth, this does NOT mean water which is changed frequently. This is because of the logarithmic nature of bacteria numbers. Going from a billion bacteria to half a billion bacteria with a 50% water change isn’t going to be that much help to the fish. Using over-filtration to go from a billion to a million bacteria will help a lot. Large water changes cannot compensate for poor filtration.
Further analysis of this bacteria can be found at this link:
4.3. Bacteria in the Water Column
Myths About Diseases
There are many myths parroted on social media about diseases. Here are just a few of them:
- Testing water parameters is essential to diagnosing fish diseases.
- Fish absorb medicines through their skin and gills.
- Freshwater fish drink the water they swim in so putting medicine into the water gets the medicine into the fish.
- The use of “chemicals” such as copper, metronidazole, praziquantel, and antibiotics to treat fish diseases should be avoided as they will probably kill the fish.
- Salt (sodium chloride) added to the aquarium assists in the healing of injuries, promotes the formation of slime coating, improves gill function, and kills some parasites.
- Using antibiotics in the home aquarium will result in antibiotic-resistant bacteria.
- Frequent water changes help cure fish diseases.
- Ich is a horrible disease that will kill most of the fish infected with it.
- “White-eyes” is due to water quality issues.
- If fish get red blotches on their skin and fins in an aquarium it is generally something called “ammonia burn”, not bacterial hemorrhagic septicemia.
- Herbal, “organic” and “natural” remedies (Melafix, Pimafix, Garlic) are effective treatments for fish that are diseased.
These myths are ALL false.
Diagnosing Fish Diseases is VERY Difficult
One caution about diagnosing fish diseases. Even with a microscopic examination fish diseases are not easy to diagnose. Most hobbyists have only what their eyes tell them to go on as to what any fish disease is. This means that diagnosis of any fish disease is simply a “best guess”. And these “best guesses” are frequently wrong. So keep this in mind in all the following disease articles.
If one has a disease break out, these chapters can hopefully help:
10.2.5. Chilodonella and Costia
10.3.10. Red Mouth in Goldfish
10.7. Fish Saprolegnia or “Fungus”
10.16. Graphite Disease in Bettas
Some general articles will be useful when treating any fish disease:
12.2. Various Treatments Summarized
This is in addition to a chapter on symptoms. These articles are as follows:
11.1. Hole-in-the-head Syndrome
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