If you're using T-12's, the usual distance is about 8". With T-8's or T-5's you can increase that distance. Ultimately, let the plants tell you. If they're reaching for the light, move it a little closer.

I would only buy T8 fixture. My preferred one is from Home Depot - the cool tubes are something like $8 a piece.
I remove the wraparound thingie, and attach a cord. Some of them come with cords, some with wires ready to be attached to the ceiling box. I usually save the tri-prong cords from old fixtures and rewire them.

Yes - I side with Mark - 12" distance for this one.

Irina

Here is a link that might be useful: wraparound t8

Thanks for the info. The light is an older tabletop light, with grolux tubes in it. Can't find out if it is T8 or whatever. I used it for my mature plants, but bought a 48 inch tabletop becuse the plants got too big for it. So I'm using this for a nursery and miniviolet light.