I tend to agree. It is true that the existing leaves on a sword will often yellow and die off during the first couple of months, as another member explained. But if these are leaves that have appeared since you got the plant, there is a nutrient issue. The latter could still be relevant even with the original leaves too, just to be clear.
The API root tabs have caused problems for other members, I would not use them. I believe they can also be very messy, so try not to disturb what are already in the substrate.
What exactly do the dying leaves look like? Can you perhaps post a photo when you can log in from your PC? It would help to see the issue.
What is the light over the tank? Be specific--type, number of tubes/bulbs, wattage and kelvin. And the tank size (length and gallons).
The bubbler should not be necessary, unless the tank is way overstocked there will not be a shortage of oxygen, and while it is true that this can drive off much-needed CO2 i agree this is not likely to be so fast. Though this could be one of several factors contributing.