Please include information about what kind of plants you are trying to grow, how many watts per gallon you have, etc. How are your plants dying? Pinpoints of algal growth on the leaves or do they simply yellow and die? I would definately stay with live plants; They're so beneficial.
First off, they oxygenate the water. Second, they absorb Nitrate, a chemical that would otherwise build up. It can be taken care of with fake plants with regular water changes but is kept in check with live plants. Third, they compete with algae for the same nutrients from the water, so since plants are far more demanding, they usually hog all the nutrients.
With all these benefits, what is the disadvantage? Lighting. You have to be willing to take care of these plant's needs. With a 55g, you have two 24" flourescent lamps at 15 watts a piece. If this mirrors your current setup, then you most likely have about .5 watts per gallon (30 watts/55 gallons = .54 watts/gallon). Not to worry though, my friend is keeping his 55g with the same lighting setup and his plants are steadily growing; at a slow pace albeit. With your setup, you should be able to maintain plants, but they wont flourish.
As for CO2 fertilization, you definatly dont need it at this point. You should wait to consider that until you have vastly more lighting. As far as my knowledge of that process goes, your plants are not growing rapidly enough (lower light) to require excess CO2 to be availible.
Hope this helps with a few of your problems!
