How would it make emersed plants worse? It gives carbon to plants in a chemical form that is similar to the chemical plants extract from C02. It is not however anywhere near as effective as actual C02. First comes light which generates growth, next comes C02 which fuels photosynthesis, then comes Macro nutrients which are vital for photosynthesis to work and transport nutrients throughout the plant, and then comes micro nutrients. Elevated C02 levels can make up for low light levels, but if you have neither, your plants are in trouble and you can dump all the ferts in the water you want and the only thing that will grow is algae!
I was speaking of the expression of the nutrient deficiency. One of the advantages of adding CO2 or other carbon sources is faster growth. This is even true in low light environments. In fact if you spend some time reading Tom Barr's ideas adding CO2 reduces the light needs as the plants are better able to maximize the use of the light they recieve.
As the carbon triggers the plant to grow faster the nutrient imbalance will become more apparent. Plant symptoms and outbreaks of algae are all very common in this situation.
Balancing out a planted tank can be a tricky situation. Matching your plants, lighting, stocking, and fertilizer can take some trial and error. Don't get discouraged.
I'd visit some of the forums dedicated to planted tanks to learn more.
http://www.barrreport.com/forum.php
http://www.aquaticplantcentral.com/
