Most diseases occurring in fishes are the result of severe stress and a diminished immune system caused by environmental factors.
If you suspect an illness with your fish, a single specimen outbreak is best treated individually, and this article shows such treatments suitable for individual cases as well as whole tank infections. It is recommended that a hospital tank be kept handy, containing nothing more than a sponge or other biologically and mechanically based filtration method. Exclude any chemical filtration such as carbon.
If your entire tank is affected, the biological filtration will most likely be compromised during treatment, so monitor your water quality frequently. Before beginning treatment remove any chemical filtration (such as carbon), try to pinpoint the cause of the outbreak and fix the problem while performing the treatment. This may entail massive water changes if the problem is water quality related, but you cannot cure the disease without curing the causing agent as well.
If an illness occurs in your aquarium, please check the following and correct appropriately, while undergoing treatment:
1. Temperature: Has there been a sharp rise or drop recently of 4 or more degrees Farenheit within 24 hours? If so slowly adjust temperature no more than one degree farenheit per day till optimum temperature is reached.
2. Water quality: Is there a high ammonia(>1.0 ppm freshwater and >0.5ppm saltwater) level in you tank? How about pH levels (<6.6 community freshwater and <7.8 saltwater). Salinity in saltwater system (<1.0018-1.0028<) Nitrites exceeding 1.0 ppm. All of these factors outside the recommended parameters wear away the fishes' slime layer making them prone to disease outbreaks.
3. Unsuitable tankmates: Any torn fins, missing scales, or other signs of aggression in your system?
4. Improper acclimation. Did you acclimate your new fish SLOWLY to your tank, allowing for temperature and water quality changes? Did the fish appear stressed at the fish store? The best preventative for introduction of disease is a quarantine of 30 days in a separate quarantine tank, but not all of us have the opportunities to do it (but think about the benefits, especially when introducing marines).
5. Water changes: Did you remove chlorine/chloramine when performing a water change, and make the temperature of the new water similar to that of the tank sytem? No sudden pH changes?
6. Life-support system: Are your filters and aerators working properly?
Hopefully after checking through the above checklist you have found an answer to your problems, but it doesn't always work out that easily, sometimes the cause of the illness is not found. Nonetheless, there are treatments available to the hobbyist for most conditions, and below is a list of symptoms, diagnosis and treatments suitable for, commonly occuring ailments in freshwater fish:
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Freshwater Fish Diseases
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