what kind of lighting do you have on the tank. If you don't have at least VHO lighting, that's the reason your LTA is shrinking, it's dying. LTA's are photosynthetic, and therefore need a lot of lighting in order to process. This is also probably the reason he's not eating well.
Saying "fine" for water parametes is not good. I need to know the exact levels. If you don't have testing kits, you should buy some soon, as this is crucial to keeping a healthy tank. "fine" for fish might not be "fine" for an anemone. Did your fish die all at the same time, or was it kind of a "domino" effect (they died like a week apart). After they died, did you have the water tested? Did you remove them right away. My hypothesis is that one fish died, and spiked your ammonia, which caused the next fish to die, which caused the next fish to die, etc.
Sorry if i sound mean, i'm not at all!!!
I just want to help you figure out what's going on in your tank!
Jenn