Why is my iPhone battery health decreasing day by day?

What is the issue? How fast it is degrading?

How old is the phone?

Most of the times, degrading battery life is entirely normal behavior. Batteries are degradable products and they will degrade with use and charging cycles.

See this user tip I wrote about this particular subject for more reference

About battery health on your iPhone - Apple Community

What exactly is your battery health issue? How old is your phone? What is the current maximum capacity?