hi, we have a water tank in the garden in london, england 2' (feet) by 4'
x 2' high that overflows into a smaller water tank 1.5' x 2' by 1'5 high.
in the larger tank are two goldfish and in the smaller tank just some water
plants.

we have recently found some extremely tiny fish in the small tank. they are
very similar to ones in a stream that runs about 100 yards from our house.

since we did not put them in; the question is how could they have have
possibly got in there?