No you don't need any extra lights. It would be a good idea to get a different bulb so you don't support so much algae growth but you don't have to do it.
the kalvin rateing has nothing to do with wattage your bulb would remain the same just make sure the kalvin rateing was up there. I suggested 6500k for you could get tehm at almost any HOMe Depot / Lowes etc. cheap.
The problem with going that low is a) you can promote algae growth a little more and b) you will have a more yellow light and not the crisp white light that looks nice.