I’ve never had much success with dedicated pond pumps; they are invariably limited in power and at the first grain of debris, cease operation.
I have a small garden pond with a capacity of around 4,000 litres and I use one of these:-
http://www.screwfix.com/p/titan-ttb582pmp-400w-automatic-dirty-water-pump-240v/65041
The problem is that it is too powerful at 400W, ideally I’d want one rated at 100-200W (but such things don’t appear to be available). I operate the pump via a timer for only 15 minutes a day, which is adequate to aerate the water and keep the water clean; the cost of the electric works out at less than 2p/day.
A further problem is that the pump is supplied with an earth leakage trip integral to the mains plug, which requires resetting with the loss of power. I chopped this off, fitting a normal plug, supplying the power via an isolating transformer (for safety). If you remove the safety plug supplied, make sure that there is some earth leakage or isolation protection within the installation.
To stop fish being sushied by the submersed pump, I placed it within a bucket with netting placed over the bucket top.
The pump instructions state that it is not suitable for use as a pond pump; but it works OK for me.