Historically, thin leaved plants have served as edible herbs. Some plants don't fill the bill, but when it comes to planted tank terms, it can be an indicator of an insufficient diet. A high protein diet alone can result in fish eating your plants. While this may not be completely indicative of the problem, it could be something to consider. Gouramis are bad about this, along with loaches (eating plants).
The other issue might be that your plants' current growth rates aren't meeting their potential (considering that fish may want to nibble). That's where you come in. The faster plants grow, the better; however I will warn that quality is greater than quantitiy. Maybe you should upgrade your CO2 before you upgrade your lighting. Lighting is easy to increase, but there's no substitute for a good supply of CO2 since it effectively increases the potential of your current lighting.