Not necessarily, it's a big tank to be dosing easycarbo... but co2 can be expensive and cause even more problems...So I wouldn't recommend co2 to fix problems either.
How are the plants struggling? Pics will be best if possible? Do you dose any nutrients or anything else?
That may be, but it'd be helpful to see a pic, that would help to see what plants you have, what plant mass, the state of your plants i.e. if there are obvious deficiencies, the amount of fish etc...