I use a 175 watt MH over a 36x18x24 deep tank and can grow most plants. For a 48" long tank you might want to consider two lamps to get an even pattern. A pair of 150 watt bulbs would give you more than enough light. BTW, I've tried 10,000K, 6,500K and 5,500K lamps, the 6,500K works best, IMHO. Tom