Long tentacle plates are very very hard to keep long term. Any kind of tear or rip in their flesh and they seem to contract infections easily and go south fast. I tried once and won't be trying again any time soon. Keep them on the sand, don't let annoying fish like clowns near it (I'm about ready to turn my clowns into sushi) and bascially baby the hell out of it.

As for the regular fungia plate corals the secret I have found is to feed them .... a lot. They are photosynthetic but without feeding some chopped seafood several times a week they tend ot go into decline.

One thing, even if they died dont toss the skeletons, put them in the back of the tank where you get some light and you will eventually have baby plate corals grow from the skeleton.