The theory is that the flow of water through the filter should be five times the tank volume per hour. 180 x 5 = 900, so you should hava a turnover of 900 litres per hour.
But your tank won't have 180 litres of water after you've allowed for the substrate, decor etc. And your pump won't have a turnover of 600 litres per hour as the majority of manufacturers quote flow rates without any media in the filter, which slows the water flow down (Eheim is the only manufacturer I know of that quotes flow rates with media inside the filter). If these two factors cancel each other out, you are getting a turnover of 3 times tank vol per hour.
But I read somewhere that Juwel believes that if the water flow is slower it means the water is in contact with the bacteria that little bit longer allowing for a more thorough removal of ammonia and nitrite. However, I've never heard of people that have 10 times tank vol per hour having any problems, and I have heard a lot of criticism of Juwel filters.
I would seriously think about changing the pump. Looking at Juwel's website, it seems like the rio 125, 180 and 240 all have the same pump, and the 300 and 400 have the same pump. The 240 would be seriously underpowered with the same pump as a 125!
I know it says in the manual that you can change impeller to get a higher flow rate but I think that's a misprint. On retailers' websites they say that a given impeller is for a pump of a certain size, not that you can fit a given impeller in any pump.
Or did you mean changing the pump?