Rivercats is right, it's not hard you just need a steady hand...but yeah I have heard of people using dividers (I plan to use them also) what some people did was they cut up some of those little plastic pots plants come in and use them-you just bury the plastic under the substrate on the line you don't want the plants to spread, or as Riverctas said her "imaginary line" and bury the plastic just so it's barely visible from your view under the substrate, but don't have it so its like a centimeter above the substrate...if that made sense. This just blocks the runners from spreading into the other areas. But, you could just carefully remove the invading plants as Riverctas said, but with such a deep tank and considering how fast these plants grow will probably be an inconvience to you. B. Japonica is a beautiful plant, I WAS going to use it in my iwagumi tank but I went with DHG because it went better with my scape, but in such a large tank the japonica would also look very nice...and yeah, both of these plants need lots of light, especially being at the bottom of this tank.