i guess ur lucky that you haven't seen them before. once they start breeding they are almost impossible to get rid off.
had an out door wine barrow with them in the attempt to get rid of them all i drain the barrow, scrubbed it clean, leave it to dry for a couple of days, soaked in 1% bleach solution and now two weeks later they are all back living in the water with the bleach [my guess been out in the sun all the bleach is gone]
they are fine until u want to breed egg scatters in what their in as they eat all the eggs :roll: