Are these Aptaisia? and harmful or peaceful?

[Subnautica]

I've had these boys in my tank for a year. They keep multiplying. The only fish I lost were a small flasher wrasse and small firefish (they mysteriously got wound on their body and died soon after). I'm not sure if those deaths were related to the aptaisia though. Other than that they don't seem to bother anything. But again I just want to make sure if they're harmful.

 

[WalkerLoves_TheOcean]

Yes, it looks like aptasia or majano.

I know that they can cause issues with coral, but I am not sure about fish.

I feel like no, because in our tanks we put anemones and coral with stings just as bad or worse as aptasia.

Now that I think about, I used to have a yellow clown goby that would always hang out near a rock with aptasia, then one day his pectoral fin was gone, then he died a few days later. Maybe it was the aptasia...? I have no idea.

 

[DED65]

Looks like Aiptasia to me. I only know that they can sting corals and small seahorses, I don't know if they can sting fish. It would make sense that they possibly could...

 

[Pistondog]

I've had these boys in my tank for a year. They keep multiplying. The only fish I lost were a small flasher wrasse and small firefish (they mysteriously got wound on their body and died soon after). I'm not sure if those deaths were related to the aptaisia though. Other than that they don't seem to bother anything. But again I just want to make sure if they're harmful.

More of a nuisance than danger. Aptasia can quickly overrun a tank.
I use a kalk paste in a syringe to cover the polyps.

 

[Lavey29]

That's aptasia. They do nothing to fish but will be your worst nightmare.