suddenly sick

Hello, I am a pretty new reefer, moved tanks in March and this system was thriving with the cycled rock and water, and now my tank crashed and has been withering away over the last week. All zoa polyps closed, xenia died back and shedding, kenya trees dying, birdsnest and monti dead, mushrooms...

So I just saw my YWG 2-3!days ago his belly looked full due to eating pods and his color was bright yellow then he hid for a few days and I see him today and he looks imacitated and pale, I checked all parameters and they are fine, what could be wrong?