Pest maybe?

[Jamesisjamess]

Can any one help identify these that showed up in my fatjer in laws tank

 

[Tahoe61]

Hydroids and yes they are pest. If you can remove the the rock they are on I would, if not try taking underwater epoxy and make a pancake out of it and cover Hydroid colonies as able.

 

[Wolf89]

Yes hydroids. Kill them as quick as possible!