With that much light and no CO2 you are just going to be feeding the algae. Either that or your plants are going to run out of nutrients (CO2). You need some source of carbon. It is the building block of all life and just like other fertilizers, it is a nutrient that plants need. If you put that much light over the tank you are going to make the plants photosynthesized faster. During this time they are going to need even more nutrients, than if you had them in lower light. Once they run out of these nutrients they stop growing and then algae takes over. Whether it is flourish excel or CO2 injection, you need some source of carbon for the plants to uptake. Here are a few sites explaining this further. 
http://aquaweb.pair.com/forums/archives/loach3/index.cgi?read=31686
I've got a few other tanks as well, but the pattern I've noticed is that at just over 1 watt per gallon to about 1.75 watts per gallon you can get healthy plant growth without the need for CO2. The plants don't grow as fast. With 2 or more watts per gallon, you are going to require CO2 injection because the plants will use up the CO2 in the water and then the CO3 causing pH swings. Plants furthermore won't grow as well because they are nutrient starved (CO2). Algae is more 'flexible' in it's requirements and will take over when the conditions for the higher plants are not optimal.
http://www.faqs.org/faqs/aquaria/general-faq/plants/
CO2
This is very important to plant growth. Without sufficient quantities
of dissolved CO2, photosynthesis cannot take place. Most tanks will
have some CO2 due to fish respiration but this is usually not enough
to get "lush" growth. Some plants do not need much CO2 and some plants
like Cryptocorynes actually seem to do worse with higher levels of
CO2.
Typical levels of CO2 in a non-CO2-injected aquarium are in the range
of 1-3 ppm. Most plants will flourish with levels of 10-20 ppm but
this requires some type of CO2 injection. With lower levels of CO2,
the plants will not be able to utilize high levels of light and
nutrients and the extra light and nutrients will be used by algae.
http://aquaweb.pair.com/forums/archives/loach3/index.cgi?read=31686
I've got a few other tanks as well, but the pattern I've noticed is that at just over 1 watt per gallon to about 1.75 watts per gallon you can get healthy plant growth without the need for CO2. The plants don't grow as fast. With 2 or more watts per gallon, you are going to require CO2 injection because the plants will use up the CO2 in the water and then the CO3 causing pH swings. Plants furthermore won't grow as well because they are nutrient starved (CO2). Algae is more 'flexible' in it's requirements and will take over when the conditions for the higher plants are not optimal.
http://www.faqs.org/faqs/aquaria/general-faq/plants/
CO2
This is very important to plant growth. Without sufficient quantities
of dissolved CO2, photosynthesis cannot take place. Most tanks will
have some CO2 due to fish respiration but this is usually not enough
to get "lush" growth. Some plants do not need much CO2 and some plants
like Cryptocorynes actually seem to do worse with higher levels of
CO2.
Typical levels of CO2 in a non-CO2-injected aquarium are in the range
of 1-3 ppm. Most plants will flourish with levels of 10-20 ppm but
this requires some type of CO2 injection. With lower levels of CO2,
the plants will not be able to utilize high levels of light and
nutrients and the extra light and nutrients will be used by algae.
