I've had a 250W Mogul six inches over an 18g tank running about 5 hours a day with no issues (other than heat). As long as it doesn't get splashed, you should be fine. Are yours Mogul or DE? Pendants or retros in a canopy? In addition to shortening the photo period, you could also start the corals lower in the tank and move them up as they become used to the light.