However, I find this often misleading, as a lower output will still serve well in smaller tanks that are considerably more shallow, and even a high output will fail your plants if the spectrum is not correct.
For standard fluorescent bulbs, the common rule of thumb is to aim for 2 to 4 watts per gallon, and this is generally not a bad rule to follow if you are working with standard fluorescent bulbs and low- to medium-light plants. However, I find this often misleading, as a lower output will still serve well in smaller tanks that are considerably more shallow, and even a high output will fail your plants if the spectrum is not correct.