I think it is something like 10 times the literage of the tank? Dont quote me there are more experienced people out there that can guide better. But for fully stocked for a 65 litre you should be having 650 lph? (Hope someone confirms this)
So for instance for my 160 litre I needed 1600LPH, so I have internals Fluval 3+ and Fluval 4+ which give me 700 lph and 1000lph for the full not overstocked tank volume I have (I think!)