If the pressure, weight is applied perpendicular and doesn't push out sideways with strain on the kit seams a tank can support pretty heavy load.
I gave it a try once above my pond, with a little 30 liter tank and placed it upside down only supported on it's side panels and sucked the water in with vacuum. So that was 30 liter = 30 kilo skybox filled with water for the goldfish to swim above pond level, resting only on it's side pannels, stood there for a few months and now i'm still using the same tank as test/propagator tank.
So a light fixture of a few kilo's wont hurt as long, as said, the whole construction is level and steady.. 🙂