You will want to be careful doing that. It needs to be big enough to have differential temperature so the water moves, therefore aerating it. Without air in the water nothing but mosquitos can live or breed in it. Also, depending on the soil and whether you are above the water line, you may not be able to keep enough water in it between rainfalls.

I'm sure you can look up how to do one properly, I just want to to be aware it's not as simple as dig a hole and fill it with water, because that will do more damage than good.