Light position, how high can i put T5 lighting above waterline? From lowest gravel point to water line is 43cm/430mm/16.9291339 inches How high can my lighting be above the waterline before it starts to loses effectiveness from the gravel mark, growing hc/jap hairgrass for carpet. Also, I have brought two timers for the tank. Light: Starts at 3pm, turns off at 10pm every day. Co2: Starts at 1pm, turns off at 10pm every day. Is two hours enough time to pump the co2 to the max before the lights turn on?