Two standard 20 gallon tanks are 24x12x16 and 30x12x12. Because the heights are different it takes different lighting fro the two. I assume you have a 24x12x16 tank, since you have a 24 inch light fixture. Unless you want to use CO2, and dose all of the various fertilizer elements the plants need, you will want to use low light, or around 20-30 micromols of PAR. With a light that sits on top of the tank the best lights that will do that are 2 bulb T8 lights, and converted lights using 2 or 3 screw-in CFL bulbs. With the CFL bulbs you can adjust the light intensity by using different wattage bulbs.
If you use the T5HO light, and it will work with only one bulb, which many wont, you will need to suspend the light about 12-16 inches above the top of the tank. Or you could use two bulbs, and put a layer of fiberglass window screen over the bulbs, and hang it about 14-18 inches above the top of the tank. Or you could put two layers of fiberglass over a one bulb T5HO light and maybe get by with it sitting on top of the tank. T5HO isn't really a good choice for a low light 16 inch high tank.
You can use whatever bulbs make the tank look like you think it should look, but many people like to use a 10000K and 6700K bulb together.