Do you have room for a HOB
fuge? Even if it hangs on your sump? If you could do that then you would be golden to grow whatever you like in it, but if it has to be in your DT and you don't want to worry about it goin sexual or takin over chaeto is good. It grows really fast, and is a good snack for herbivore fish. Also one that I've noticed no one has mentioned is red leaf mangroves. They can be placed in full saltwater tanks and grown off salt water. They eat excess nutrients in the tank. You just have to spray their leaves with RODI
(or your fresh water top off source) 1-2 times a day. And make sure the stay above the water with roots in water.