The condition known as congested fins is easy to confuse with fin rot, and can be particularly hard to correctly identify in fish who have red fins.
Congested fins lead to the tail and fins taking on a torn and ragged appearance, without any external trauma having occurred to cause the tearing.
While any of the fins or the tail of the fish can be affected with the condition, it is most commonly seen in the caudal fin before progressing to other areas of the body if left unchecked.
In serious or protracted cases of the condition, the fins will begin to split vertically, and are highly likely to fall prey to a secondary attack via fungal infection.
In advanced cases of congested fins, the Cyclochaeta parasite may also be present, but it is not thought that parasitic infection causes congested fins to occur in the first place.