[i have crabs]

4 bulbs wont cut it for most sps even in 18", i have 8bulbs and i just ordered a giesmann infinity to replace them with,
i would sugest a fixture with a 250w and 2-4 t5's or maybe 2x 150w and 2-4 t5's

if you really want to keep it all t5's then look at the ati or some of the other higher end lights that you can get at least 6-8 bulbs