Three, one for every 2' section. MH is good for an area about 2x2 feet. You'll need to experiment with how far above the water they need to be, I started mine at 12" and have moved down to 8". Be wary about placing them directly over any supports as the heat will melt plastic. Double ended bulbs require a UV shield as well, not familiar with that pendant but it probably has a sheet of UV blocking glass between the bulb and the water. The only other thing I would caution is to be sure you keep a spare bulb on hand as many places do not stock 6500K HQI bulbs.