Many ponders are finding that they reproduce too quickly and eat any other small fish. This may not occur with everyone though. All my mosquito fish died after two winters. Drill a few holes in the base of the bottle to let water in, and tie a string to the base so you can pull the trap in. Put this in the pond where it should float.
Fish can swim in through the bottle neck but have trouble finding the way back out.
Even if they are native to the area, adding more will increase the stress on local fish and amphibian populations. They can take a range of pH's from about 6.0 to 8.5. They also can take a variety of hardnesses. As with most livebearers, a moderate hardness and some added salt can be beneficial. Mosquito fish are known for their ability to survive in almost any collection of water outside.