Easy Life range of liquid aquarium plant fertilisers are extremely good quality and used by many aquascaping experts. Easy Life Easy Carbo is a very popular product that gives plants the correct level of CO2 that they need to thrive.
Easy Life are that fussy about getting it right that they pack the additives in black bottles to stop light penetrating the bottle and affecting the composition. They all come complete with a measuring cap so you can dose the correct amount into your aquarium.