Hydroids are known to be difficult to remove
Here are some things to try
If you can bleach or boil your rock
Burn them off
Kalk paste all that you can get
Copperbands and peppermint shrimp have been known to eat them but generally only if nothing else is available to eat first
This is a marathon so expect a long fight it is very rare that you win this in a sprint
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 My aquarium is nothing but a smorgasbord for my cats.....
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