Metal Halides. A flashlight with a 150 watt color specific bulb could probably put out the same PAR spectrum as a MH before it hits the water; it's what happens after it hits the water. Then again it depends on what kind of clam too. Maximas, Squamosa, Derasa, Gigas - T5s will most likely do. Crocea, No. If you could post what clade of specific zooxanthellae the clam has, that would help......