Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
What species of fish do you have?
Where on the body are you seeing this change (ie fins, flank, tail, etc)?
Does it looks like vessels?
Is the red discoloration severe in one spot or is it diffuse?
Can you take a photo of this to let me see what you are seeing? To post a photo, you can use the paper clip button on the tool bar above the text box. Or if you cannot see that on your phone/computer, then you can post them on any site (ie Flickr, Photobucket, Imgr etc) and paste the web link here for me to have a peek.