my fish is turning black?
ok, i know this question has been asked before, but i wanted a rather particular answer, so i thought i’d throw mine out there also..
i have 3 fish, one orange comet, one red cap fantail and one zebra danio. the tank is about 6 weeks old, so it’s just coming out of the break in period. i’ve done some homework.. the water was quite cloudy, however frequently i was doing water changes. i noticed yesterday that only one out of 3 fish (the comet) has started turning black around his fins and gills, and light spotting on his body, where the other 2 look just normal. i googled a lot of stuff about it, and looked on yahoo answers as well, and they all say ammonia burn. why would one fish get this and not the others? another website said that some pet shops dye their fish. sounds a little far fetched, but is it possible that my orange fish is actually black, and just trying to show his true colors? any help would be appreciated. in case it helps, his attitude didn’t change at all. he was still swimming normally, and still is now. oh, also, i did an almost full water change yesterday, because i freaked out that i was poisoning him 
he’s still acting normal though.
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put a little salt in the water
kill it will die soon any way
fish suck
dont panic, my son had a black gold fish, it turned orange, it took a few months, but it did not hurt the fish, dont know what it is, but he is ok, did not affect him at all.
Ammonia burns are the most likely culprit. Sometimes one fish will get them while others won’t. Your fish is not dyed, don’t worry. Also, young goldfish change color often, and black markings can appear/disappear without warning. How big is your tank? I hope it’s at least 75 gallons…
Danios need to be kept in schools, and although they can live in cooler water like goldfish, there’s a chance they’ll be eaten as the goldfish grow.
if the color transformation isnt uniform such as the top is turning black and slowly traveling down its body it is probably due to ammonia burn and you tank wasnt cycled properly thus the cloudy water. Not all fish will react the same to its environment its like sticking a hundred people in 30 degree weather with just a shirt and pants some may say its just fine and some will say its freezing. Some will catch pneumonia and die and others will live perfectly fine. Ive had seen a friends black goldfish turn into solid silver over the course of 2 months but it was a gradual thing at first he had a silver belly then his sides became silver and then he was completely silver only i noticed it because i will net out fish and hold them in my hands to exam them every couple of months in a pond.
The black is healing from ammonia burns and that is all. He will revert back to his old color in a few weeks. Not all fish are affected the same way so no worries. Petshops do dye fish but only specific ones. Goldfish are not one of those fish. They cannot be dyed. He is orange and will be orange.
NO ALMOST full water changes. Once per week 25% water change with cleaning and that is it. You are going to force your tank to recycle and subject the fish to the cycle process all over again.
Test the water weekly, clean and do a 25% water change no more. Your fish will live fine and healthy.