Here is some options for T5 lighting to look at. Also there is metal hallide lighting if you want to go there. T5 6 bulb is a good way to go with your 18 " tank height for growing just about anything. Here is a link to our t5's for sale.
http://www.sunlightsupply.com/produc...s%3D%26pgi%3D2
Thanks
Kevin