Most generally a 24 x 24 footprint for each MH light, to get good coverage.
you can skimp slightly on this but not a lot. 72 foot would be 3 lights,
UNLESS you want some dark areas at one or both ends for softies. then you
could get away with 2 MH, then had the "caves" in the dark areas.

I personally had 3 400MH in a tank roughly 72L x 40H X 28W a homemade, built
in plywood "350 gal". With the sump it was almost 400 gallon of water. but
I had to leave it when I sold my house and got married. I miss that tank

Steve


"N9mxt1" wrote in message
...
How many of them would I need for my 125? Its 72" long

Jeffrey