Look "20 inch light for planted aquariums" lots of sites.

Recessed Full Aquarium Fluorescent Light Hood, 15 watt, Size 20 Inch x 10 Inch.
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The Marineland LED Aquarium Hoods are state of the art light hoods. The s give the same shimmering effect as Metal Halides. The low power consumption and long bulb life make these sleek, low profile hoods more economical to operate than their fluorescent cousins. Available in 3 sizes which fit most 20, 24 and 30-inch sizes.
The Aqueon Modular LED Aquarium Light Fixture, 20 inch, takes LED lighting a step forward
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The special note here is that the bulbs accepted are now 24 inches long for the 20 gallon long tanks. For the light bulbs remain 18 inches. Because you are able to select your own bulbs there is no light issue with plants. New bulbs range from $5-$12 depending on the quality and can be expected to last between 6 months and a year. For longer bulb life keep the bulb well protected and clean it regularly. When both LED and flourescent hoods do not meet your needs you're probably ready to take the next step in aquarium lighting and fixtures. I'm talking about lighting strips. Both suspended and resting above a glass top. These two options are used in serious fish stores world wide as they are cost effective and provide even lighting. 18 Inch T5 Aquarium Lighting Fixture 20 Inch T5 Aquarium Lighting Fixture 18
Photo provided by FlickrAquatop Skyaqua 20 Led Aquarium Light Black 20 Inch - SJKAquatics
Photo provided by FlickrJan 15, 2017 - Aqueon LED Aquarium Light Fixture, 20-Inch – image
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The Marineland LED Aquarium Hood has three sizes to choose from - 20.5 x 10.2 x 3.6 inches, 24 x 12.2 x 3.6 inches and 30.5 x 12.2 x 3.7 inches, allowing you to fit it on most standard fish tanks. Made of durable plastic and glass it also features a great night light effect.
These fixtures are the latest fluorescent technology yet. They measure only 7 mm and allow for several bulbs in a small space. A 13 watt 20 inch T-2 Bulb (6400 K) produces 950 lumens which is 73 lumens per watt with low wasted green/yellow light energy that is often found in other Power Compact Lights. As little as 1 to 1.50 watts per gallon for a planted aquarium is all that is needed from these T2 Lights!
These fixtures have useful linkable, and rotating lens features too.
The T5 has become very popular among both plant keeping freshwater aquarists, and reef keepers for good reason. They are compact, come in many varieties and high lumen per watt outputs. As a broad generalization, only requiring 2 to 4 watts per gallon for more shallow planted/reef applications depending upon tank depth and other factors. While the T2 is the latest in straight tube fluorescent lights, the T5 still is better suited for aquariums over 20 inches in depth. Generally around 13 mm in diameter. This is a mini pin bulb which generally uses even less watts per lumen than many than T-8 bulbs. T-5 lamps range from 65 to 105 lumens per watt.
The T5 became very popular in the early 2000s among both plant keeping freshwater aquarists and reef keepers for good reasons; they are compact, come in MANY varieties and high lumen per watt outputs (as a broad generalization only requiring 2 to 4 watts per gallon for more shallow planted/reef applications depending upon tank depth and other factors).
While the T2 was the latest in straight tube fluorescent lights, the T5 still is better suited for aquariums over 20 inches in depth. The T-5 also comes in many more different varieties, sizes, and even "qualities" (such as the premium Giesemann), and thus is the preferred option for most