he means 80cm., which is roughly 31.5 inches. a metal halide covers a 24" x 24" area, so a single lamp is definitely going to leave you with dark corners. how deep is the tank from top to bottom? you could buy a 250 watt or 400 watt pendant (depending on the depth of the tank) and raise it up high enough to cover the entire thing, but you are going to have a lot of light spilling out around the tank.
as leo stated, you could go with a t5 HO
fixture, but you would either be using a 30" fixture which only has 24" lamps in it, or a 36" fixture, which will hang over the sides of the tank a bit. i would probably use a 36" fixture. the 36" fixture's lamps are only about 34" anyway, so you aren't going to have a lot of light spilling over.