My Tetras are turning brown?!

[Canno2827]

I have 3 cardinal tetras they were doing perfectly fine and then when I went to go check on them one was hiding in the log and had a luminescent brown stripe instead of blue. Will it die, is it breeding or do I need to change the water please help!

 

[thomasina]

If you can put a picture up. How long has it been since you changed water and have you tested your water?