I don't feed my peace lilies at all, but if you insist, I suggest Vigoro or Mirace Gro 'blue water' type food, at half strength.
I rarely feed my potted plants. I try to always use rainwater for watering.
Do make sure the soil is not staying too wet. But in my experience, they can take a lot of wet soil.
I can't really advise, You may do better to post a question in the Houseplant forum. Good luck