A few drops of vegetable oil on the water will supposedly help deter
mosquitos (can't breathe the air through the surface of the water - this
isn't compatible w/ the fish solution. )
Read about Gambusias, but have never seen them for sale. Supposedly very
tough, can take bad water conditions. If you can, get a temp of the water,
the guppies might be able to take it even if the air temp is dropping at
night. Feeder guppies are still around 10 for a $1 (US).