Carpet plants need good CO2 to do well. With that, they don't need nearly as much light. But, T5HO bulbs give about 30-40 micromols of PAR per bulb at 28 inches distance, so a 2 tube fixture can give enough light for most plants, including carpet plants. A 4 tube fixture is getting to be very high light and can lead to algae problems if you are not really fastidious about tank maintenance, water cleanliness, fertilizing, CO2, etc. at all times.