The nudi you are refering to isnt actually a nudibranch. Its called Chelidonura Varians, and poses no threat to your zoos. The problem with CV is its diet is very specific, it only eats planaria flatworms, and it needs alot of them to survive so what ends up happening is it eats the flatworms until there's a very small population then it starves and dies leaving a few to repopulate if it doesnt get sucked into a powerhead first.
A product called flatworm exit gets mixed reviews, I've used it a few times and always there's a few that survive to repopulate. Also the worms are toxic and release their poison when they die so if you kill too many at once they can nuke your tank.
Now I just rely on regular siphoning in conjunction with a water change. I had read that eventually their population would drastically reduce or they could completely disappear on their own, and I believe this has happened for me. I used to siphon them out frequently at least a couple times a week. Now its every two to three weeks and even then there are hardly to remove.
So i would recommend patience and regular siphoning.
