Err, depends on the type of plants. It helps to look up on some of these things, but most of the time the big thing is lighting. If your tank is old, then you may need to replace your bulbs as well; 6 months to 1 year means your bulbs will lose wattage. You want 2 watts per gallon.

Otherwise for prettier plants, you may need to get liquid fertilizer. I also use a CO2 enhancement for the tank.

On a side note, usually floating plants can do all right in your tank. Just remember that lighting rule.