Ok I have gotten some great help on here from some of you..I have a new tank from Christmas and I got 2 feeder fish,goldfish because the lady at petco told me i need them to cycle but I know now I didnt need them..ok..one of the goldfish is great and looks great..but the other one is turning black and it stays near the heater most of the day...is it burning itself? what do I do? and Also...how do I clean the tank by changing the water?? I dont want to mess up the temp with the fish but I need to know how to take some old water out and put some fresh in without killing the fish...I am getting Glofish next week and I want everything to be perfect for them...

I had a black one that that turned orange. Feeder fish also. Some of them turned out to be fantail goldfish.

DO NOT GO TO PETCO!!!!
ive been there a MILLION times everytime my fish have gotten diseases and died
BUT I also got those goldfish they ALL turned black got lumps on their heads and bred with eachother so many times we had to get rid of some
NOTE we had a tank the size of a bathtub no joke!
they were getting mutated and gross
i then killed them all cleaned the tank and got fish from a local pet store- no problems
many fish ive gotten from petco have gotten diseases and died!
DO NOT go there!

personal expperience

Goldfish and koi change color as they mature or in response to the environment/food. It's normal. The lady was lying. There's this thing called a fishless cycle. You don't need the goldfish. What you you plan to do with the goldfish after the cycling is over? And by the way, goldfish are temperate fish. As in, they are meant for cold water. So turn off the heater. They don't need it. And haha, some fish like to stay near heaters to steal warmth. Careful that he doesn't burn himself.