Ack! NOT a bruise.
It really sounds like columnaris to me; is the number one infection in fish tanks. I grabbed this from
www.fish-disease.com:
"Columnaris (a.
k.a. Mouth Fungus)
Symptoms:
An infected fish will have off-white to gray cotton-like patches on the head, fins, gills, body and particularly the mouth. In
time, these areas will develop into open sores. Gill swelling may occur, gill filaments may stick together and excessive mucus may develop in the gill area.
Rapid breathing can be seen. Fins may deteriorate to the point of leaving the fin rays bare. Muscles may be inflamed and capillaries may rupture. Fish, particularly livebearers, may exhibit "shimmying". Infection may be acute (killing an infected fish within hours), or chronic (lingering for several days before eventually killing the fish).
As with most diseases, not all symptoms need be present.
Cause:
The bacterium Flexibacter columnaris.
Treatment:
Ensure that your water conditions (e.g., Ammonia, Nitrite, pH, Nitrate levels, and water temp.) are within their proper ranges. If not, perform a water change and/or treat the water accordingly. Recommended medications include: Furanace, Fungus Eliminator, Fungus Cure, Furacyn, Furan-2, Triple Sulfa, E.M. Tablets, Tetracycline, or Potassium Permanganate. Medicated foods are also recommended. Columnaris can be highly infectious and may quickly kill all aquarium inhabitants; therefore, early treatment is essential. All fish, including those not yet showing visible symptoms, as well as the aquarium they inhabit should be treated."
I don't know if we can catch it in time; sounds like an acute case (since things are happening SO quickly). Any of the "cyns" will hopefully help (Maracyn and maracyn2, Kanacyn, Furacyn); you'll unfortunately need to treat the main tank too as its pretty virulent. If you can find medicated foods and he is willing to eat, even better. If not, theres a recipe for medicated foods here:
http://www.fishdoc.co.uk/treatments/medicatedfood.htm . I had another site with other directions but can't locate it. Damn.
Grab some Bio-Spira too if you can; the meds will play havoc with your biological filter. As soon as the meds are cleared from the tank add it - it should help kick start the bacterial colonies.
*slaps self in head* Yeah, a heater would be necessary unless the room stays at over 80f. Can't believe I forgot that. However, check the tank temp, for 2 reasons. One, if theres a tank light it may keep the tank water at a decent temp and 2, I've read you DON'T want high temps for fish with bacterial disease (like you do for ich), as it may help it propagate. I don't know how true that last bit is; I mean, when we get an infection our temp goes UP to help fight it, but thought since I've seen it mentioned I should pass it along.
*edit*
Hah! Found it:
http://aquariumhobbyist.com/artoffishkeeping/disease.html
