Sylvania makes a candelabra bulb in a "daylight" color which uses less than 6 watts. I use one bulb to light a 20g tank adequately. Two would be better. Four might be too much. The water sparkles very nice too. It's like a mini metal halide. They cost about $5 each. The socket is about the same, a cord, a switch, ... Wish I could buy it premade!
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I've converted my old incandescent aquarium fixtures by using a socket adapter. Very easy. (I attached a picture with two bulbs over a 58 gallon.)
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After building a fixture having a dozen of these bulbs to light a 54 gallon corner aquarium I found it was definitely too bright when all the bulbs were on at the same time. The cost for that fixture really added up too! In the end I found two bulbs were plenty.
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