Hi Roxie.
Im no expert with ponds (only got mine going about 3 weeks ago) but if i tell you what im using,maybe that may be of some help to you.
My pond (pictured below)is about 10 ft. long by about 7ft.wide and 32 inches deep (approx 900 gallons i think)
I have a pump called an "Atom 3000" placed on a block of concrete about 4 inches off the bottom of the pond.
This powers the water feature (they supply 3 different ones with the pump).
From this pump,i have a length of tubing which runs to the filter (called a Bermuda 7500)this is a pressurised type of filter which enables me to place the filter about 1 metre lower than the waterfall.
From this filter,i have a length of tubing running up to the top of the waterfall where the water then trickles back into the pond.
( I understand there are 2 types of filters you can buy....a pressurised one like mine.......and a normal filter which i believe you have to have above or in line with the top of the waterfall)
I personally prefer the pressurised type as you get a better water pressure going up to the waterfall.
I hope this helps you,and i apologise if any of the information i have given is wrong (ive only just really started myself)
Maybe some of the more experienced members would like to comment on the above.
Best of luck Roxie