First, watts is irrelevant here because all fluorescent tubes come in basic wattages according to the length of the tube, and the type. There is some variation between manufacturers but the tube that fits your fixture will only be in the wattage for that tube. For example, a 48-inch T8 tube is 40w or sometimes 32w, depending upon the manufacturer; the newer 32w tubes are more energy efficient (using less power [watts] to produce the same light). A 48-inch T5 will be 54w. And so forth. But the type (T8, T5, T12, etc) being different means that the light produced by these various tubes is not going to be comparable on the basis of watts.
To your fixture, it will take T8 tubes, and apparently T10 and T12. The T8 are newer and better. [T5 will not fit.] Take one of the existing tubes with you to be sure, but the same length in a T8 should do it. For planted tanks, most of us recommend "daylight" type tubes with a Kelvin of around 6500K. If you want inexpensive tubes, the GE, Phillips and Sylvania T8 tubes having 6500K work fine. Just look for the proper length in T8, with 6500K. The manufacturers use various names, but the Kelvin is the guide.