Retro is when you buy T-5 reflectors and ballasts and endcaps separately and "bolt in" to your hood.. This will actually be cheaper than buying a full fixture..
By hood I mean like a tall like canopy that has room for lights..
like this
I have two lumenbright MH reflectors (250w bulbs) with two T-5's. one on each side.
If your gonna put T-5's in
Got to www.customaquatic.com and check out there T-5 retor kits
I would get 4 bulbs worth of reflectors and then get yourself an Icecap 660 ballast
and buy 4 sets of endcaps.
and bolt them on..
is your tank 4' long?
if so get the 4' bulbs of course.
you'll get good light and be very happy.