This is true, especially if the tank is moderately to heavily planted. Activated carbon will remove a lot of elements from the water that are plant's nutrients. So this is why people that have planted tanks don't usually use activated carbon.pauliface said:They are healthier. How ever I read in my fluval (external filter) manual that if you have live plants in the tank, that the carbon filter will counter act the plant. So i guess if you are using carbon filter, then live plants may be something to avoid, try looking more into it o get more advise here.