Darryl

Follow up questions:

1. Exactly what kind of soil did you use for repotting?
2. Can you show me the fertilizer you used? Was it a liquid? Was it granules? Did the instructions say to mix 1 tablespoon per x volume of soil?
3. When you say you used reverse osmosis water, are you referring to the RO water that you drink or some kind of waste water that may come from the system?

Darryl’s Analysis

Thanks for the additional information!  Here are a few of my comments/observations:

1. Watering practice: RO water is totally fine for plants – some professionals even prefer using it over plain tap water.  The issues you are facing are in both aspects of watering – the “when to water” and the “how to water”.  WHEN to water: a peace lily should be watered when its soil is around halfway dry, not when completely dry.  HOW to water: ideally, the soil should be thoroughly and evenly soaked while letting excess water drain away.  Pouring small amounts of water leaves dry pockets where roots can die off.
2. The soil you used says “Garden Soil – For In-Ground Use”, which is much too dense and water-retentive for a peace lily.  Houseplant soil will consist of peat moss, coco coir, and perlite (and maybe bark chips) – or you can look for a bag of soil that says “for houseplants”.  This is the main reason your plant seems to have suddenly collapsed after repotting.
3. As you noticed, the instructions for your fertilizer said “1/2 teaspoon” for small pots – so 1 tablespoon is WAY too much.

At this point, the plant is likely too far gone to be revived.  In the future, make sure you’re using soil that is appropriate for houseplants – peat moss/coco coir is light brown and fluffy; perlite looks like little white pebbles.

If you want to learn good foundations in houseplant care to avoid costly/heartbreaking errors, I think you’ll enjoy my book or online course.