You are now at the point where you have a nicely sized tank for growing plants and having a nice number of fish as well.  Unfortunately your tank is rather deep and the lights that come with tanks are normally 20-40 watts.  At this stage you may need 100 watts or more of light depending on the depth and length of your tank.  Additionally there is growing debate on the relationship of tank size vs wattage vs water hardness.  Obviously if you have a water hardness higher than the plant would like it will need more nourishment (fertilizer, CO2, and light) to encourage it to keep growing or it will die off.  Of course there is a limit to the ability of survival as well.  Typically they say 2-3 watts per gallon for normal softness of water for fish.  This would mean a 40 gallon tank would need between 80 to 120 watts of light.  Also if you have floating plants or those with large leaves toward the surface this will block light getting to the plants toward the bottom of the tank.