If you are still designing your pond then I would definitely go for a gravity fed filter and not use a submersible pump.
Most of us start off with just a few koi, and in time we buy too many fish and those we have grow rapidly and our ponds are soon over stocked. The filter is the most important part of your system and you will regret using small filters. The experience of Steve is something that you hear about all the time http://koicymru.co.uk/ponduk09g.htm Part 2 of this story will appear in the next couple of days, and you will see he decides to fit a larger filter which is not easy as you now have limited space in the filter chamber. It is important to get it right the first time.
The number of airstones you will need depends on the media in your filters. If you have a venturi in the return to the pond this will be sufficient air in the winter, and you can add a couple of stones in the summer. A 40 litre air pump is probably the right size. Many people go for a 80 litre but you have to be very careful with these as if you only have say 6 airstones you can cause the pump to run hot and it will fail and they are expensive to repair. Many people have had this problem.
Quite a few people have a bottom drain with an aerated ring and these usually need their own pump.
Posted on Dec 13, 2006, 7:38 AM from IP address 81.129.89.67