right.....but you can't put a compact fluorescent bulb in a regular fixture. You can only use a standard fluorescent tube, which is likely 15-18watts. And 20 tall and 20 long are the two standard sizes of 20g tanks (24x12 or 30x12 with one taller than the other) There is also a 20g extra tall which is 20x10x24t, but that's not likely what you have. How long is your tank?