Magnifica are nearly impossible to find healthy. They usually succumb to bacterial infection. I doubt a water change will help, unless there is a problem with the water.... might make things worse....
I have never had an anemone get stressed from high nitrates or phosphates, BTW. Now pH, salinity, ammonia, nitrite, and temp.....those are more important.... If those are screwed definitely do a water change...
Don't move it to QT unless that QT is filled with antibiotics..... Why stress it out more?