Pinch that bitch cathar
I just tried out my cheap chinese (resun) pump ,that has similar specs to your pond master , through a 4mm dia nozzle, it got 3.1L/minute, I was just about to test it with another smaller pump in series (like what was suggested for you) when my large pump broke it's intake almost clean off!
however before it met it's premature end I did notice the flow through the nozzle was considerably greater.