You really should go to the User CP and complete a tank profile so we know about your tank.
Your initial post tells us nothing about how big the tank is, or how long the tank has been setup, what other inhabitants are in it and worse of all you stated that your water parameters are perfect! Well, what are they (post real numbers from quality liquid or digital test kits) please.
How long have you had this fish?
Did you QT it, and if so for how long? How long ago?
What do you feed, and how often?
What is the tank filtration and biological filtration?
How big was the water change that you did?
Did you match temp, SG
, etc. before changing the water?
How did you do the water change?
The end result may still be MFD (mysterious fish death/disappearance), but at least we may be able to rule out other possibilities.