dieing coral

Context: 3 gallon aio Only other corals are a small Duncan. Had in there since may and he was doing great opens up every day and looked amazing. Then I wake up this morning and I see this. One of the head is just gone. Bear with me I am new to the hobby but is it just closed or completely dead...

Around 2 weeks ago my completely healthy large wall hammer as well as one of my three torch corals started a slow decline. I assumed it was water quality due to a the fact my rodi filters were due to be replaced but after replacing them the quality for the corals mentioned only declined. I...

My montipora is dieing. What can i do to help?