Pest Or Harmless

I've had this saltwater tank running pretty smooth for a while. I have a 65 gallon display with 2 clownfish, 2 BanggaI Cardinal, 1 melanarus wrasse and 1 blue green chromis. I had a diamond goby but last week he was acting strange like he was trying to jump and kept breathing hard. He dipped into the rocks and I haven't seen him since, so maybe this is part of that. I went down to feed the fish and noticed this small, flat, round black thing on the glass. I have no clue what it is so any ideas for what it is would be helpful

 

That looks like a stomatella snail from that angle.

 

That snail wouldn't be the cause of the issue with the diamond watchman goby. I have had 2 DWGs and neither one of them made it a year and had similar behavior to what you described, but my other fish have been fine. I switched to conch snails and have been much happier with them (they don't create sandstorms in the tank or piles of sand). They also do eat detritus and other stuff that the DWG won't. I have 5 of them in my 90 gallon with a 60lb sand bed and they keep it pretty clean.

 

Conchs are a favorite of mine in my tank also !
The snail in your pic is not a pest.

 

Conchs are a favorite of mine in my tank also !
The snail in your pic is not a pest.

Ah okay thanks everyone for the replied