As mentioned before, add some quick growing floaters or other stem plants (wisteria, rotala, Stargrass, guppy grass, Vals. etc.). You do have way too much light for just the moss and the swords you got growing in there right now. You don't need to add "a CO2 system". Just get some fast growers in there to suck up those nutrients. Keep up with the Excel/CO2 Booster as well.