What if you first use a specific type of water for a few weeks, and then use another type of water?
Plant watered with purified water-- health began to decline. At first, it was watered with tap water.
Certified GKH Gardening Expert
Depending on the plant, it may be experiencing a chlorine deficiency. Many plants benefit from tap water, as many nutrient supplements are not full supplementation. They may lack certain elements like chlorine or fluorine, which plants use both in VERY SMALL quantities.
While it is true that certain plants have a sensitivity to it, there are also many plants that need it.
What is the plant in question? Maybe we can work through the reasons that this could have happened.