Cut it off. Before it reaches the surface, preferably.

Zenkeri will completely shade your tank if you give it half a chance. Cutting
the leaves off before they reach the surface will encourage the plant to put
out submersed leaves. Once it "knows" it can reach the surface, it will
produce only surface leaves. It is possible to re-train the plant to produce
submersed leaves again, but it's better to never let it reach the surface to
begin with.


Leigh

http://www.fortunecity.com/lavender/halloween/881/