I've always used Clorox, or any chlorine laundry bleach will do. But....you need to do it every week or so in the wet season. Also, just spraying some Baygon blue or green down there will help. Pete
EDIT: Blue is their mosquito spray. Green is stronger for bigger nasties...but works great on mosquitos as well.
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If it's standing water, a few drops of oil works as well, it floats on the surface and stops the larvae from breathing. You can use vegetable based cooking oil or similar, you don't have to empty your sump down the drain!