This is fin-rot
Although the tail is damaged, this isn't fin rot
Don't mistake split or torn fins for fin rot. This Ram is NOT suffering from fin rot, although small injuries to tail and fins could allow the infection to gain a foot hold if the fish are under stress.
As its name suggests, fin rot causes the tail and other fins literally to rot away. If caught early enough then an over the counter remedy will most probably treat the condition. On the other hand if the infection is well established or seems resistant to shop bought remedies then antibiotics may be needed.
One of the primary causes of fin rot is keeping the fish in a poor environment, which could mean any of the following - unhygienic surroundings, wrong temp, wrong water chemistry, where it is subjected to fin nipping, over crowding and so on. The cause will need to be addressed before any treatment is started or the treatment is likely to fail in the long term.
Goldfish and other coldwater fish are more at risk because waste takes longer to break down in colder conditions than in tropical conditions, if goldfish are kept over crowded then waste builds up faster than it is broken down which may lead to stressed fish and a steep rise in the numbers of background bacteria present, a recipe for ill health. Fish which have been bred with artificially long fins are also more at risk because the fins are more prone to suffering from minor injuries and the fin extremities can suffer from poor blood circulation which can impede healing. For this reason fancy goldfish with long flowing fins shouldn't be over wintered in outdoor ponds in cooler climates.
A split fin or even a frayed fin doesn't necessarily mean fin rot. If the fin tissue looks dead and breaks away quite easily or if the edge is white or fungus is visible on it then it is more likely to be fin rot.
Fungus on the affected area is a common secondary infection and can be used as a indicator to diagnosis. If the infection is just on the edge of a fin it should respond quite well to treatment. However if the infection has reached the body of the fish prompt treatment with antibiotics may be the only course of action likely to succeed.
Prompt treatment is essential with fin rot because established infections are very difficult to treat. Early symptoms should be treated with an aquarium remedy made specifically to treat fin rot. In most cases this will treat the disease. Established infections may need treating with antibiotics under the guidance of a Vet.
The following have been used successfully to treat fin rot.
It is important that the fish should be slowly acclimatised to this solution slowly by building up to it over a few days rather than suddenly changing from fresh water to salt water of this strength.
Is easy by keeping the fish in a good clean environment which suits that particular species and with a good maintenance regime in place.
Early treatment is essential, once established fin rot is almost impossible to treat with shop bought remedies.
Don't rush out and treat every split fin as though it were fin rot, minor splits and tears heal with out any help.
Use a proper remedy and not one of the many popular herbal or homeopathic remedies which simply don't work.
Don't ignore any bullying in the tank