They are good scavengers and show up in most tanks. Numbers relate to detritus levels. I remove larger ones when I see them.
When they get large even the most common types they can irritate corals or even harm some fish.
One morning, I found one of my one inch seahorse juveniles attached to a rock on the bottom of it's rearing tank with it's head down; not a good sign, I moved the rock since it's tail was partially under it; it skittered toward the surface with a 2 inch bristle worm attached to it's tail. After a bit the worm let go. The seahorse was lethargic and not eating well for a few days before the the bottom quarter inch of it's tail turned white. and ultimately fell off. It recoverd but has a stumpy tail that didn't regenerate.