The answer is BOTH.
Pumps/filters help keep the water clear,,,,, and help keep the fish alive. Although you could easily have one without the other, You could have perfectly clear water that is not capable of supporting fish life, or you could have cloudy green water that fish are perfectly happy in.
You can think of your proposed pond as a big aquarium, and if you look at any aquarium with fish in it, how many do you see that don't have some sort of pump or filter? Of course there are also goldfish bowls, they don't have pumps or filters, but they keep live fish in them and quite often the water seems clear????
It all seems very confusing doesn't it?
The first aquarium I ever had when I was about 8 or 9 years old I filled full of fish I netted out of the local creek. I must have had 100 or more little minnows swimming around in that 20 gallon tank with no filter or pump or anything, I remember how cool they looked swimming around in there, and bugging my parents to take me to the pet store to buy some fish food before they all starved. They promised they would do it the next day, but by that evening it was too late nearly all of them were belly up, and by the following morning everyone of them was dead. I was very upset with my parents that they didn't take me to get that fish food right away because in my little child mind I reasoned that they had all died because they starved to death.
Of course the truth of the mater was very different, but it took me years to understand all the things that went wrong.