I took apart my Pico tank tonight to siphon out all the detritus. My seahorses are in a hospital tank, so all it contained was rocks, zoos, a dendrophyllia and a cleaner shrimp. I pulled everything out and picked off any fat bristleworms, as well as any razor caulerpa that continues to spread and annoy me. (Larger bristleworms are not good for seahorses, I've been told.)

Then I found one of these. Plucked it off while being angry I'd not seen it before. Then as I added stuff back into the tank, I found a larger one!!!!!!!! It too is down the drain.

These eat zoanthids, if you didn't know it. I'd not seen one in my tank in months, but the zoos have died off a little bit at a time. Looks like I found at least one of the reasons.