Im going to order Slimline Metal Halide Floodlights to light my tank but dont know if I should get 2 of the 75w versions or 2 of the 150w versions. Difference in cost is about £20 so the only question is will 300w with pressurised co2 and EI be too much over a tank of dimensions : 100W x 40D x 70H cm.

IMO 150W would be too much and 300W ridiculous. MH lights are intensely bright and therefore there would be no need for that kind of wattage. Would be incredibly hard to manage the tank on dosing, CO2 and pruning with that level of lighting.

You could easily get away with 1 of these lights wattage wise! 2WPG max IMO.

Out of interest why are you looking at MH instead of fluoro tubes? Its not as if it is that big a tank. Fluoros would be much cheaper.

The days of people 'needing' 4,5,6 WPG are long gone. I put needing in quotations because even in the days of T10/12 I doubt that these levels were needed. Technology moves on and efficiency gets better. Tanks with 2WPG T5 are IMO high light these days whereas some people are asuming they can use the WPG rule for them.

MH are probably even higher than T5 and in no way can you use the WPG rule with them.

Andy