Some attention needs to be paid to correct sizing since they can run up a power bill a surprising amount. Vortex or non-clog impellers can be used. Some basic bar screen is a good idea. Fish tanks and ponds should have a pump capable of recirculating the entire tank or pond volume in 2 hours. Very small waterfalls, such as indoor ones, are not our area.
Outdoor waterfalls can have vastly different water requirements.
A rule of thumb is 50 GPM per foot of width. Water features, such as streams, take large volumes and very low lift. The friction in the return pipe is the major element and should be calculated. The flow will vary with slope, shape, width, and roughness. Please don ' t use these numbers for multi-thousand dollar projects without getting some professional help.