FWIW .. I am dealing with a TERRIBLE valonia infestation in my 20g right now. What seems to have stemmed the tide is an introduction of 3 or 4 emerald crabs and a green urchin. I have a green urchin in another tank and that tank is valonia free (although it has hermits so I wasn't 100% on whether it was urchin or combination urchin/hermits to thank for that).
Anyhow, about 6 weeks on, I'm noticing significantly less valonia. The bigger ones don't go away but I am able to remove those manually (or the urchin uses them to decorate himself).
I'm not 100% sure of the latin id on the urchin but this has been one of my favourite algae controlling purchases I've ever made (so much so that I now have 2.
). He does wonders for filamentous algae too. Not sure if he'd go after caulerpa too, but I should see if I can find a caulerpa infested rock that I could try an experiment on.
Anyhow since I can't give you an id to help you if you want to find one, here's a picture:
If you see one of these, try it. Only thing as a decorator urchin, be prepared to have to glue down EVERYTHING in your tank that you don't want rearranged. Zoanthid frags are a favourite...
PS. I'm also eating my words from the past that I've never had emerald crabs deal with valonia. I do have them now and they are indeed doing their job (I can see them). The problem is that one crab is probably only good for removing so many valonias in a time period .. and so if the valonia can replace themselves faster than the crab can eat them, you have a problem. With the case of my 20g, it seemed that I needed a good 3 or 4 before the tide was turned.