I can only assume like the previous poster that you have the fluorescent light fixture but no "cover" for the tank that would support the light fixture. What you are looking for comes in 2 types.
1: full plastic hood often sold as an assembly with a light fixture (such as that which you already possess)
2: a "glass" type flat cover. This is often called a glass canopy. Glass canopies are essential aquarium equipment that provide a protective layer between your aquarium and light fixture. In most instances a 55 will require TWO of a 20/25 gallon cover. Some companies supposedly sell a full length 55g unit. I would avoid risking this due to a 55 gallons cross brace unless you could verify it is just a "kit" IE 2 x 20/25 sold together. *WARNING* either way this will be expensive! I would estimate you are looking at anywhere from 60.00-100.00 for glass and can guarantee you are looking at the very least 80.00+ for 2x the full plastic hood/light assembly. I know this as cheapest 20/25/55 I found was walmarts hood and light assembly for my 20 (also fits 55 but you need 2) @39.99 plus tax. I just bought it yesterday.
Manufacturers page and info~
Versatile glass aquarium canopies fit most standard size aquariums
* Covers aquariums to reduce evaporation and prevent fish from jumping out
* Glass tops provide a protective layer between aquarium and light fixtures
These glass canopies reduce evaporation and prevent fish from jumping out of the aquarium. Perfect for tropical or saltwater setups, each Versa-Top has a Super-flex hinge that simplifies mealtime by remaining open without any propping or holding. Glass canopies are essential aquarium equipment that provide a protective layer between your aquarium and light fixture.
Versa-Tops glass canopies includes a clear vinyl back strip to add approximately 2" to the total width. Back strip helps create a tight seal but openings can be easily cut out to accommodate heaters, filters, and other vital aquarium equipment and accessories. Versa-Top is stylish and versatile enough to match any room décor.
Model Aquarium
Size Dimensions
w/out back Strip
16" 5-1/2 gallon 15-1/2" x 6"
20" 10 gallon 19-1/2" x 8"
24" 15 gallon / 20H;
use 2 for 55 gallon w/brace 23-1/8" x 9-1/2"
30" 20L / 29 gallon 29-1/8" x 9-1/2"
36" 30 gallon / 40H 35-1/8" x 9-1/2"
48" 55 gallon, older style
- no center brace 46-5/8" x 9-1/2"
36" x 18" 50 / 65 gallon,
requires center brace 2 - 16-7/8" x 14-1/4" panels
48" x 18" 75 / 90 gallon,
requires center brace 2 - 22-1/8" x 14-1/4" panels
72" x 18" 125 / 150 gallon,
requires 2 braces 3 - 22-1/8" x 14-1/4" panels
24" Twin Tube 15 gallon / 20H 23-3/16" front panel
6-1/2" back panel
Hope this helps