Usefullness of sand sifting cucumbers?

[trouble]

i just picked up a sea hare, and although so far has not lived up to my expectations, has definitely made sum progress on the algae. i picked him up at river front in calgary, 50$.a very interesting creature. but i have heard some rumblings of polluting the tank if they die. but after doing sum research have found out that it is related to slugs but(if it is not one)one of the few that are good to be kept in captivity, most of the other species are very picky eaters and is hard to determine exactly what they eat and there for hard to keep food stocked so they do not die from starvation. regardless of all that i have run into quite a few people that swear by these sea hares as sand sifting algae eaters.