Generally the fins will start to look thinner and like they are shriveling and perhaps like they are thinner between the rays of the fins. They can also start to look shorter. Sometimes all the fins will be affected and sometimes only one of them. Usually this attacks them when they have been in water that is too cool or if they have a bacterial infection. It is much easier to treat when the cause is from cool water, the bacterial finrot is sometimes not easily treated and can be fatal for them.
If the cause is cool water the first thing to do is make sure that you keep the fish's environment very clean and warm within the range of 78 to 80 degrees Fahrenheit or 25 to 26 degrees Celsius consistently. Temperature fluctuation is the most dangerous thing and water with ammonia and nitrites will be the next most dangerous thing as those two things are involved in causing defective fins in any fish.
The bacterial finrot will require the needs of medication to cure and if the tank is not cycled this is a very hard thing to regulate as the medication has to be administered and the daily water changes have to be maintained to eliminate the ammonia, nitrites and nitrates until the filter is cycled. It can be a very hard thing to figure medication dosages to replace the meds that are removed by water changes and add enough to make up for the next days dose. I have the tables figured for this but would need to know information for tank size and temperature and type of filter and of course the carbon has to be removed from the filter to keep it from removing the medications from the water.
I would recommend stocking Triple Sulfa and Rid-Fungus for when/if finrot occurs. Together those make a pretty effective treatment. Garlic Guard, Vita-Chem and Fish Protector are also good to have and use. Those 3 are non-harsh and safe at any time.
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