More common in goldfish than most other fish, this is a vast moving and particularly destructive disease. It is very sad to watch a fish succumb to this. It seems to just give up; it gets more and more lethargic but just doesnt die. It gets to the point where it’ll only move when poked, wont eat, floats belly-up, and just disentigrates.
The disease is caused by a bacteria that attacks vascular walls, not unlike the Ebola virus and other hemorrhagic fevers. Once established in the fish’s body, it travels very quickly through the circulatory system and attacks the blood vessel and he heart. This debilitates the fish, but take an excruciatingly long time to kill it, for it takes many days to completely destroy the circulatory system.
This causes the following symtoms:
Apparently broken blood vessels in fins and extremeties
Splotches of red, especially under the gill plates
Steadily increasing lethargy.
The only treatment ive ever read of is Maracyn-2. This should most definately be admistered in a quarantine tank, as septicemia is contagious. I had a goldfish come down with it in a tank with another goldfish and a pleco, but none of them were affected. Apparently there must be a wound or some sort of stress factor for it to get established. Irreverible damage can be caused without prompt treatment.
There is also apparently a viral type of septicemia. I have never heard of any treatment for this, and it exhibits much the same symptoms.
USGS work on viral septicemia
UFla on Catfish septicemia