AquaClear 20 impeller on the left, and the new larger blades on the AquaClear 50 impeller in the middle.
I’ve owned a few of these AquaClear hang on back filters over the years and they’re really easy to modify. Just by changing the impeller to one from a more powerful unit increases the flow from 350 litres/hour to 550 litres/hour
After a bit of research I ordered an AquaClear 50 impeller (number 634), and a spare shaft in case (I didn’t end up needing it at all). It popped right into the pump and as soon as I switched the power back on it was clearly evident that flow was up 2 fold!