Any sessile inverts that you regret (sponges, fan worms).

[Reef keeper 103]

Are there any sessile inverts that you regret And would never keep again? things like scallops are valid answers.

 

[Cthulukelele]

I'd never get a ball sponge again. They almost never thrive in home aquaria

 

[Imrahilwjz]

I know that @Paul B would say encrusting photosynthetic blue sponge (or maybe it was purple). In my old 90 gallon tank I had yellow sponges that did well, but they died with everything else when the tank crashed.