Hi Bubble, is that the zac carbo plus unit using the carbon plates you are refering to? I have one on order myself. Its another way of putting carbon dioxide for the plants into the water. Its only harmfull to the fish if the levels exceed the recommended 10-15ppm which shouldn,t happen if you only use it when the lights are on. In fact the fish should flourish because of the extra disolved oxygen in the water. There have been many tests done to show that you cannot disolve oxygen into the water via an airstone. But plants that have enough carbon dioxide can saturate the water with up to 120% disolved oxygen concentration! A eye needs to be kept on your ph if you do use it as the carbon dioxide level rises the ph will fall. Carbon dioxide is only a part of growing healthy plants though, before investing in one of those units, make sure all the other ingredients are there first. 1: Enough light of the right spectrum 2: Water movement around the roots and leaves (this varies with some species of plants) 3: An aquarium fertilizer with iron, magnesium and other trace elements. 4: Carbon dioxide from fish or other source. 5: Regular water changes to replenish trace elements 6: Talk to your plants (only joking)
Plants come from different climates and like different water temperatures, ph levels, KH levels, light levels, etc so try to get plant species that come from areas with your water parameters. Goodluck with your gardening, you,ll be a greenthumb in no time.
