Pond fountain knowledge needed

[slartibartfast]

Hi all, i wonder if anyone can help me, My Girlfriends pond fountain has melted and welded the impellor to the inside so it's a write off! not bad considering it's about 12 years old.
unfortunatly there isn't anything written on the old pump to what make or model it is so we don't know what litres per hour pump we need to replace it.

The pond isn't very big, 6 foot long, 3 foot wide by 2 foot deep, not a very big pond, so what fountain should i need to help oxygenate the fish?
The fish BTW are just small at the moment.
or can i replace it with an airpump?

We haven't got a clue but hopefully someone on here will know.

Thanks in advance

Paul