I recently built a terrarium out of a 20-High fish tank (24"W x 17"T x 12"D), and am using 3 x 24W (24") High output T-5 fixtures to give me about 500fc at the sufrace of the medium.
I would thing that an MH bulb would generate way too much heat.
Part of the problem is the dimensions, as T5's will hang out over the edges of the top. CFL's might be an option, as would some of the phosphor-type LED's.
By the way, "watts" per se, are not your concern, but the lumens output. Lumens per watt changes with the type of bulb.
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