My 90g tank uses 2x54w with inferior reflector. I can grow Crypts, easy stems & swords. If I use 4x54w I can get robust growth, wider variety of plants and some nice colors.
But he pointed to retrofit kits... I'd assume put them end to end in a 6' housing?The problem is that the fixture is only 36 inches long and the tank is twice that long, so the ends of the tank will be very dimly lighted. Adding a second fixture would solve that, but leave you with too much light, unless you could leave some of the bulbs turned off. You could also raise the 36 inch fixture a foot or so above the tank, to get more even lighting along the length of the tank, but then the light intensity would be too low for almost all plants.
Now I'm not sure just what you get with that kit. The illustration shows only two bulbs and one ballast, but the description refers to 4 bulbs. If what you get is two of the kits shown in the illustration I agree that mounting them end to end would work fine.But he pointed to retrofit kits... I'd assume put them end to end in a 6' housing?
Light level notes are dead on - I run 8x39 TEK over my 180g, and it runs pretty high light.