I keep getting email from this thread saying there are new posts, but then there an't any new ones. You HAVE to stick with a t8 bulb with your current fixture, you cannot make it better by simply putting in a T5-HO bulb. It's a completely different pin arrangement, and requires different electronics to light it up.
There is nothing you can do to that fixture to make it better for coral. Either upgrade, or try to squeek by with really low light corals.