flurish excel is a carbon suppliment. you can use it with your co2
but it's kinda along the same idea.
i did a search for plant gro and the stuff i found has .15 Nitrate along with some trace elements. if you look at your bottle they usually have some numbers like .15 - 0 - 0. the first number is nitrate, second is phosphate, third is potasium. those are your macro nutrients and you have to have all of them in your tank. if you bottom out on one of the nutrients your plants cant utilize the other ones. and since there is nutrients that the plants arent using, algae will use them.
and your swords need root tabs. some say theirs do fine without them but swords are root feeders and mine die without them. give them a try, i bet they rebound in a few weeks