New Tank: What Are These.... Things On The Glass!?

[adam9175]

Hey everyone!

After several years of freshwater, I've got a new 200 litre saltwater tank. I've had 40 kg of live rock in there for 3 weeks and its crawling with copepods. I also have loads of these...... things on the glass. They're about 2 mm in diameter and what started as one I could see a week ago has turned into at least 30! What are they???

 

[ainsy]

They are the medusa stage of a hydroid.

It is impossible to tell if they are good or bad as there are literally 100,000+ types. They can reach plague proportions so if I were you I wouldn't take the chance and remove them.