They look the same superficially but are very different-
Osmolator 3, 10 minute pumping time limit, safety sensor
Osmolator 3 Nano ,77 or 154 second pumping time limit (only for nanos, will time out on anything over 65 gallons), no safety sensor, the safety is the time limit
Same optic sensor system on both.
The Osmolator 3 Nano is built to be reasonably priced for nano tanks where the overall investment is lower. The cables are off the shelf cables, one for pump, one for power, the Osmolator 3 has a custom made Y cable that is made in Germany and Japanese made connectors for pump and power. While the pump is the same, the cable is different and a further cost cutting is that instead of push fittings for the hose connection it has a barbed/lock nut style fitting.