Why my green plant leaves turn brown? I did switch on the aquarium light at least 12 hr per day, mine is a 13 gallon tank, with two 15 watt fluorecent plant tube. So do any plant experts know why does this happens? Lack of CO2? ( but its impossible, I have lots of fishes in the tank and only 5 pots of plants. ), I did a 20% water change every week, so the water should be alright. Btw, those plants already survived for around 3 months, its was healthy last month, but all of a sudden, they turned brown. But not all the 5 pots turned brown, only two of them turned. Btw all are of different species.