I use two of these fixtures on my 6 foot 150g:
http://www.bigalsonline.com/BigAlsU.../allglassfluorescenttwintubestriplight36black
they are well made with full polished aluminum insides for max. brightness.
they would work very well on a 180g too. I did replace the bulbs with Hagen Powerglos rather than the more pinkish Allglass lights that come with it, but some people love that look. you will have to see what you like.