Boiling it gets rid of nutrients so no, just weight it under a rock. Only try a little at first and the ordinary round lettuce is best. You could also try cooked spinach or defrosted ready cooked, again only a tiny amount. My guppies eat about a 20th of one leaf, but you can always eat the rest yourself.
Iceburg lettuce is pretty much just water, and useless. Leaf lettuce is best (boston, romain, spinach, etc). I use a suction cup clip to stick it to the back wall, sort of looks like a plant that way.