Now that, is an open ended question.
Lots of options. Cheap or High end LEDs, T5s, Halides, or a combination. And it depends on what you want to keep.
If I was starting from scratch on a 180, I would probably go with either a 5 foot 6 bulb T5 fixture, or 3 x 250 watt DE halide, or maybe halide + a single pair of retrofitted T5s. If someone made me go LED, I would probably either get 6 black box or 6 Noopsyche K7s.