Goldfish & Koi

Question
Two weeks ago I put two new young koi in the tank, and within 48 hours two young goldfish were dead, the two black moors were losing thier color, one bright red comet is covered in black smudges/spots with black smudges on the tail of the fantail goldfish. The two large (6-8") fancy goldfish were at the top of the tank positioned vertically and the rest were all huddled on the ground in corners. The only time they would leave those positions was to "flash" every once in a while.  All were holding all fins in TIGHT, tails and fins are now ragged like something ate them, they wouldn't eat, were obviously in trouble.  The koi, however, are just fine (go figure).  I moved everyone to a "hospital" tank, lots of aeration, low water level, salt and a fungicide.  All perked right up - fins erect, swimming and EATING !!!  I then proceded to clean the original tank - new water, rinsed the gravel profusly (could have filled the swimming pool with all the water I used).  I heat sterilized the artificial plants, and shells, decorations.  I set the tank up to run through for a few days - the PH levels are perfect.  BUT - here's the problem - I don't know WHAT the poor fish came down with, but in this VERY clean tank I have noticed thin whitish looking strands (hair-like) attached to the plants, the shells, the sides of the tank.  What is this and how do I get rid of it?  I have put in salt, and put the heater in on high.  I don't want to lose any more fish.  What should I do to the environment, and what is the black staining on my red comet goldfish and the tail of the fantail? Will it eventually go away?  Would you have any idea?  Sorry this is so long.  I hope I didn't miss anything, though.

Thank you so much for any insight at all.  

Jeanne

Answer
Hi Jeanne:  What you have described is a toxic tank... not specifically a disease.  Fish will flash and scratch on things when they itch and a chemical imbalance in the tank will irritate their skin much the way you and I get a rash or hives.  While your cleaning efforts have been valiant... they were without need.  A simple 50% water change would have perked everyone up... as was evidenced when you put them into the hospital tank.  The black marks on the red comet are not uncommon when fish get stressed... There are no known cases of black fungus in the fish world... at least not in the aquarium.  Fish have the ability to darken and lighten themselves when they are stressed and in goldfish that often results in the appearance of black spots, etc.  The koi were probably ok through out this whole process because they were more used to the water chemistry that occurred in your tank after you added them.  The long white strands of stuff in your tank now is probably hair algae or white algae which can look like stray cobwebs.  I would add some beneficial bacteria to the tank and have the water chemistry checked.  If the water chemistry is ok... then I would add the fish back into the tank a couple at a time. You do not want to dump them all in there at once as that will overload the tank and the same thing will happen again.  The water in the aquarium will need to be checked every other day for the next couple of weeks or until the water chemistry stabilizes.... You have basically started a brand new tank and it will have to cycle completely before the fish will be safe in it.  Koi and black moor should never be mixed... koi can pick out the moor's eyes during moments of aggression. I hope this helps... if you have other questions please let me know... dave