I've tried all sorts of different products, animals, etc. and the thing that worked the best was Flatworm Exit, by Salifert. The nudibranchs just die, and I own a six line wrasse, yellow wrasse and have tried various other means. I found only the Salifert product to work. I urge you to use caution though as this stuff will kill them right away, but they leave a serious amount of chemicals in the water immediately after they perish. I recommend that you use lots of carbon for a few weeks after the treatment as well, just to be sure. The key with these little guys is to get them taken care of right away before they get to the point where killing them, will kill your tank. Good luck
MM
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90 gal SPS display+65 gallon mixed reef
GEAR: Korallin calcium reactor, bio pellets, digital aquatics controller, Tunze ATO, MP40, 4 x Tunze 6105 w/controller, Reef Octopus Xp3000 ext, 2x250w MH, Futura LED.