I don't know how many flatworms you have, only that I have read many posts by people who "know" that they have way too many flatworms for one of these nudibranchs to possibly eat all of them... the nudibranch always ends up dying either to a stinging coral or a power head as Iam mentioned, or they run out of food. And if you don't think the nudi would run out of food, then they really aren't going to help you with your problem, are they? I'm not trying to be critical, just wondering if the "use" of these animals has been thought through.
Despite the numbers, perhaps it would be worth while to use thin diameter rigid hose to perform water changes -- at the same time, use the rigid hose to suck the individual flatworms up and out of the tank. Sure, it won't be an overnight solution, but it also won't mean the death of an animal that you ultimately don't really have a desire to keep alive.
Sorry, lots of people probably think that a slug should be lucky to be of service to humans as a utility creature, but I really like sea slugs and hate to see them die in tanks.