Disease identification help.

[andrewsmart]

two and a half weeks ago i got a black and white butterfly fish.

now stupidly i did not have aQ/T tank and added him to me 100g FOWL displlay.

after 1 week i noticed part of his tail was missing.

the next day he had popeye in one eye which ithought might be net trauma anyway the next day the other eye was poped.

i started to treat with melafix and after two days one eye was back to normal.

i finished the course of melafix on sunday and the other eye was still not perfect but was better. But i noticed a brownness on his snout and face. and a lightening of the black stripe.

this got progressively worse and the fish was really brown in the face yesterday and died last night.

i know this can be a sign of poor water quality. so i did two 10 water changes.

my bio load is very low i have only had a foxface and two clowns in this tank for almost a year

my trates trites and amonia are all 0
ph 8.2
salinity 35
alk is normal.

what else could be the problem i have always used r/o water. but have not run carbon for 9 months at all. i did use some tap water conditioner on the tank before adding the fish as a precaution.

i am woried about my other fish now althogh thay look fine.

i know i have 2 rougue crabs in the tank which i will catch asap they may have bitten the tail causing infection.

the fish was doing great in the first week.

could this be parasitic.

my other worry is i may have parasites in the tank that the other fish are imune to and everything i now add will die.

i would greatly appreciate any help the last week has been horrible.

i will set up a quarantine tank before i add anything more.

[andrewsmart]

any ideas or suggestons