Chaeto IME is the easiest macro available. Just stick it in saltwater and give it light. It doesn't even seem to need that much light. FWIW, a while back I had to shut my fuge down for quite some time. I took the chaeto I had growing and left it in an unlit 10 gallon tank of salt water with no circulation and no heat. After 3 months I finally got around to dealing with it; 80% of the head sized wad was still dark green, and when I stuck that green algae in my new fuge it was churning out oxygen bubbles within 2 hours. This is bombproof stuff.