That about covers it. You can also boil other leafy vegetables like cabbage, kale, collards, turnip greens, beet greens, mustard greens, etc. Algae should be soft; most tadpoles won't eat tough stringy algaes. If there is a pond or stream nearby, scoop up some slimy old leaf pack. They love to graze on the slime.
Just a note- if you use lettuce, choose a dark lettuce, like romaine or leaf lettuce, instead of iceburg. Iceburg lettuce has very little nutritional value.