Alot depends on the type lighting your using, if your using any liquid carbon (such as Excel), and how many plants you need. When you do WC's you are removing excess nitrates and phosphates along with other dissolved organics, your not adding them. Fish waste and food will naturally add nitrates and phosphates to a tank which will be fine if you are running a low tech tank with few plants. Most liquid ferts are mainly if not totally micro nutrients, which plants need along with macro nutrients (nitrates, phosphates, potassium). If you have a low tech tank and your plants look good and are growing, then you don't need to add anything else. If you have alot of plants and higher light then you may need to change your fertilizing routine. Liquid carbon and CO2 help with plant photosynthesis and growth and also helps the plants to use available nutrients in the water column better. So what are your tank specifics? Lights, bulb color temp (6700K, 10000K, ???), how long are you running your lights, what are your water readings (nitrates, phosphates, PH, GH, KH), and how many plants do you have?