You should put it 3/4 of the way down, or if you have that 175W MH in a standard 55 gallon and the light is centered then put it off to the side. I don't know how you would have a MH in the center of a 55 if there is a center brace. Mostly acans like lower lighting like ideally t-5s, as i found that to be best for growth and coloring up. Acans also can do good under direct MH, but it can bleach. Just think about where aussie acans come from.....
