If you are wondering about live plants, as far as for a bacteria source, add them before you start cycling your tank. If you are talking about a planted tank, I would think you would want to wait till your tank is cycled. I do not know much about planted tanks however. Others here would be more suited to answer that for you. I bought my potted plant as basically a sacrificial lamb, to be used only for the bacteria that it provided. Also I don't believe that plants produce any waste, in fact they consume nitrates for fertilizer as I understand.