how can you get mangroves to grow outward?

I have a mangrove growing in my 20 long mantis tank that has 8 leaves on it already and is still growing. I have cut off the growth tip several times but it just grows back. Do I need to cut it back farther? Past some of the leaves perhaps to get it to grow outward instead of upward?

Thanks for your help. I also have two more I am growing from propagates I found at the beach. They were extremely dried out and the grown tip was completely brown and broken off. I wasn't sure if they were too far gone but both have grown roots now and one looks like it is starting to develop two growth tips!

 

Good question stepho, I got 5 from John @ reefcleaners. I think 2 are dead? I'm floating them on a chunk of styro in my fuge. 3 are growing roots kinda cool. Maybe if you pm'd john? This is my first using mangroves, so it would be cool to get more info.