The 1800 doesn't matter, in fact it has a smaller resistance than a 360 (though a statement of AC, I think it's 'cause of less, but longer pipes).
Have you checked the pro for dirt? In the past, there were some people showing pictures were the pro was so full of rubish, that the flow wasm't as high, as it should have been
.
A Liang can help, even if you put it (her?) on 7-8V, but a 1048 should be at least a little bit better (I had a bigger system with a 1048 running and it had a flow of ~49 L/h - without pro, but although much more :-/).