Choosing a pump for a waterfall in a retention pond, earth bottom pond, or lagoon is similar to that of a koi pond in that you have to choose a pump that can deliver enough water to get the amount of water needed up to the starting point of the waterfall. We typically recommend external pumps for this application because they are less likely to clog and there are no electrical components in the water posing a threat to swimmers.
When choosing a pump to aerate pond water, whether it be for water quality or to sustain and improve fish health, typically a submersible aeration system or floating fountain are the two best choices.
Aeration systems are the most efficient tool to use when aerating a large pond. They use little power versus the amount of air they produce. Subsurface aeration systems consist of a compressor that sits outside of the pond and air stones, or diffusers, that are in the pond. The diffusers are connected to the compressor via tubing. The compressor quietly produces the air and pumps it to the diffusers thus aerating the pond. The only downside of a subsurface system is that it does not provide the beautiful display of an aerating fountain.
Aerating fountains are exactly as the name implies. Any quality aerating fountain is going to float on the water surface. This prevents the motor from clogging. Aerating fountains typically consist of the float, motor, propeller, and an optional light kit. All aerating fountains that float have to be anchored down or tied to the shore. Aerating fountains are typically sized 1:1 with regards to horsepower and pond size; i.e. 1 1/2 acre pond = 1.5 HP aerating fountain.
When sizing a subsurface system, any reputable manufacturer will have size recommendations for each system.