Hi
I've got Bob in my water butt (named after Bob Flowerdew - the idea of keeping goldfish was found in his book. Had him/her for two years and he/she has tripled in size.
Bob certainly does his/her bit with the mozzie larva. Due to the rather Heath Robinson type guttering and downpipe from my shed I can't keep a lid on the butt where he/she lives so always leave it at least half full and use the water from the other butts that feed off this one.
To make the whole thing look even more outlandish it has branches in for anything that might fall in (no casualties so far) and I float a chunk of polystyrene on the top for when the water freezes.
Sue