top placement is normal. you can usually point them down a little bit and thats usually enough. if you find you are getting dead spots, then adjust so you can get them covered.
there are many things that can play into this. if you are using a
sump and an overflow box or something then surface isn't as much of a concern (cause its goals are accomplished by the overflow) and you can keep them lower (or pointed lower) in the tank.
but, if you dont have an overflow its important to keep good surface movement to break up excess protein build up and promote gas exchange..
hope that helps
if you have more question let me know..
