What is the shape of the patch? Is the location of the patch on his body, tail fin, or anus? Is he eating?

Some reddish patches are signs of a bacteria infection (usually the roundish or rectangle shaped). It could also be a wound if he is fighting with someone or got injured somewhere in the tank. There is a fungus that causes streaking like veins.

Those are just a few....posting a pic...even a bad phone pic can really help.