Oh... Iām sorry for the typo. I meant fluorescent not incandescent. Itās a 20w 24ā bulb. My tank is 30x11x20 with a bowed front.
OK, that narrows it down. A fluorescent tube 24 inches in length at 20w will be T8 fluorescent (there is also T5, and maybe another one or two, they are not interchangeable). T8 tubes come in standard wattages for the tube length, so you cannot get a more intense tube in T8, other than by selecting a well-made one that does put out more light than cheaper tubes. The absolute best tube here is the Life-Glo T8. I have tried many of the available tubes, and this one does put out more intensity for the tube, and it is exactly the needed spectrum (6500K). Some fish stores carry these, or you can get them online. Just make sure it is Life-Glo, T8, 24-inch.
That is your best option with the existing fixture, so I will just say that even this will be low to slightly moderate lighting. This brings me to the tank size...if those measurements are in inches, and even with a bowfront, I would expect the volume to be closer to 30-35 US gallons and not 55g. ?? Moving on, I have this tube over my 29g which has the same length width and height but is rectangular. But the depth is the issue, so this is comparable. Anubias should be fine, I can manage this in my tank. Java Fern is another low light plant. Both attach to wood or rock so they can be used decoratively. Amazon swords will be trickier; I can just get the pygmy chain sword to manage without thriving under this light. I did have a standard sword several years ago in one of these tanks, and it too "managed."
I also have floating plants, which cuts down the light of course, but given the remarkable benefits of floating plants to the fish (they do not appreciate overhead light) and the water quality, they are worth it. True floating plants that are substantial are best, species like Water Sprite, Frogbit, Water Lettuce.
The above concerns the present light fixture. If you want to get a brighter light, probably in LED now, that is another option.