What are the likely causes of White poop??

[2_Basic]

I've been battling this issue in my discus since I started 1.5yrs ago. I've always had an issue with a few discus in each tank. I've learned to put them in a hospital tank with metro and most of the healthier discus seem to recover. But after 1.5 yrs my tanks and discus count climbing I'm still battling this issue.

What are the main common/likely reasons why white feces symptons occur?

thanks:(