You have some options:

40 wat fluorescents - stagger them or mount them diagonally. These
are inefficient but cheap to buy initially.

compact fluorescents. The curly oens are for home use to
repace lightbulbs. You don't wantthem near your aquaria;
the proper ones are fantastically efficient and long loved but
expensive.

I'd be tempted to use 2 150W mercury lamps. They're cheap
enough to buy as night safetly lamps. Add a few incandescent
halognes to make up the red that's missing in the mercury lamps
and you're done.

There's no cheap way to properly light up a six foot tank!
--
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