The water change might have stressed it out and that is why it has changed colour.
The scratching along the gravel can be caused by poor water quality (ie: ammonia, nitrite, nitrate, etc) or by skin parasites like whitespot.
The whitespot parasite, or other parasites, might have been introduced with the tubifex, or another fish/ plant that has been added to the tank.
See if there are any little white dots on the fish, if so this is whitespot and you will need to treat the tank. If you can't see any spots then monitor the fish over the next few days and see how it goes. If the scratching gets worse then something is irritating it and then I would suggest posting a picture of the fish so someone can id the problem.
