I have a bubble coral that is about the size of a vollyball and it eats every day. LOVE watching it eat.
I usually feed my fish right over it and anyhting that they miss falls into the bubble.
I have it positioned on its side almost like it grew out of the wall. This allows me to see the mouth very well. And what a mouth it is. At least 6in across!! It almost looks like it has a great big smile. For this reason my wife and I call it Seymore (aka little shop of horrors) "FEED ME!!"
Any food that hits the feeders is moved to the mouth within about 10 seconds.
If a fish hits it, it will contract almost instantaniously.
Some times it is all bubbles,