Basically each one has its own specific dietary needs. If they are not met they will die quite quickly. Actually they die very quickly anyways as most only live a very short natural life span of less than one year. Generally that doesnt even matter as many will spend all of their time hiding in the liverock so once you put it into your tank you may never really see it again. Its too bad but none are very well suited to be enjoyable in a mixed reef aquarium.
I've had 4 so far. Two I bought deliberately, one to eat aptasia which disappeared within days and the other ate flatworms. The Flatworms eating blue velvet nudi did a great job eating 95% of my flatworms but then it simply couldnt located the scattered remaining few. It lasted about a month in my tanks which were loaded with flatworms! The other two I've had were just unidentified hitch-hikers that I saw once or twice before they disappeared.
I once found a thread on Reef Central where a person was breeding the aptasia eating variety in a small species only tank that he supplied with aptasia from another system. I know a lot of people would certainly love it if someone in Canada could do that.
Check out this link for more information.
http://www.seaslugforum.net/