If you want your mangroves to grow quickly, look up some of the papers by Anthony Calfo. I used a few of his tricks, like misting the leaves daily, and mine took off until it actually got too big to be in my tank (I had no sump at the time). It seems the trick is simulating their natural environment (rising/falling water levels, mists from the sea and rain which removes exfoliated salt, etc.) rather than just sticking them in a sump and expecting them to grow. Their just like a coral woth their own requirements.
Poke around a bit. I think he even did a video presentation on this... try his linkspage at Marine Depot (
http://forum.marinedepot.com/Topic19836-13-1.aspx)