I have some caulerpa in mine..came in as a hitchiker but since I am extremely careful with what I do with my wastewater, I don't mind keeping it. Grows like a week, of course, so I trim occasionally, and the 'horses love it!
If you're willing (and since you have horses, I bet you probably are) to do a little extra work, Gorgonia are perfect hitching posts. You just need a very small p'rhead to direct flow and keep algae from growing, and feed 'em live phytoplankton from time to time.