Long cords are not a problem but you do need to buy ones with larger gauge wire to handle the distance.
Start out by picking a pump to know how many AMPs you need to provide. Then use a table like this determine the cord needed.
What you are determining with this table is wire gauge. A 30 meter cord to pull 3 to 6 amps will require a wire diameter of ,75 mm(area of cross section of wire is square mm) or 14AWG. If money is not tight I like to jump up to the next size which in this case would be 1mm wire. A heavier cord is never a problem but on that is too light is.
The cords you linked to is only a 10A cord if you use just the one 10 meter length. If you string 3 of them together the amprage it is carried is reduced to maybe 5A (guess)..
This chart is for US cords but it gives you an idea how the amps a given cord can handle drops off as it gets longer.
Always use a GFI (or UK equivalent) where you plug the cord in.
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The other option is to put an air pump where the electricity is and run any air tight pipe between the pump and the pond. You could even strap garden hoses to the bottom of your chain link fence and run air through them. It is not too hard to do.