How long did you climatise them for.
What are your water stats in ammonia, nitrite,nitrate and ph.
Sounds like columnaris.
[URL="http
/www.flippersandfins.net/flexibacter.htm"]http
/www.flippersandfins.net/flexibacter.htm[/URL]
Taken from the above link.
Often the fuzzy patch will be located on the fish’s back, surrounding the dorsal (top) fin, having the characteristic look to it of a saddle, giving rise to the term “saddle-back lesion”. These lesions may also be found on the fins (fin rot), especially on the caudal (tail) fin and on the gills. The natural course of “Mouth Fungus” is that the white fuzzies or patches are followed within a day or two by redness, ulceration and necrosis, which means that the mouth quickly turns into a gnarly mess!
For columnaris in the uk myxazin by waterllife and pimafix.