Review on AQQA Submersible Water Pump with Controller - Variable Frequency Return Pump for Marine Freshwater, Fish Tanks, Aquariums, Fountains, Hydroponic Ponds - 800-3200 GPH by Rachel Ford

This thing uses a 20kHz PWM circuit to drive the pump. If you have a modern cordless drill or other variable speed PWM device, you are probably familiar with the high pitched sound they make when adjusting the speed. The problem is that they have a high current, so it's a VERY loud 20kHz noise that resonates throughout the tank and always whines no matter what the speed is set. You might not be able to hear 20kHz, but it'll still give you a headache once you're in the room. I've included a spectrum plot to show the high frequency peaks. This was recorded by holding a microphone to the glass of an aquarium with a submersible pump, so I can only imagine the stress/torture the fish are under. I would only use this outdoors, for applications where there are no live animals or people around.