I've had these problems in the past as well, and it's definitely a nutrient/co2 problem. I also only had the problem with the sword plants, other plants didn't really seem affected (I only saw they didn't grow as fast as they could, but they didn't die) If you've planted it quite recently the sword plant might be adapting to it's underwater state, and that way the old leaves are dying. However if new leaves either aren't growing, or dying quickly as well it might definitely be a problem with nutrients. I suppose it's best to remove the dying leaves, and hopefully that might kick-start the plant in making new leaves. Do you have more plants in your aquarium that show yellowing leaves or holes in them? Do be careful not to add too much nutrients in your water, as then the plants might not be able to use it quickly enough, giving algae a better chance to flourish.