Have you tried to grow any of the grasses out there, inside of your tanks, and did you have good luck in getting it to take off and grow?
I know that many may believe that seagrass, manatee grass, turtle grass, or eelgrass are macro-algae (and grow for this purpose), but they are actually some of the few species of plants that survive within saltwater. I remember when I was first learning about these grass types and trying to grow them, without realizing that they have some decent requirements dependent on which species you plan to have.