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Will a sterilising agent in rain water used for watering plants have an adverse effect on the plants?

The rain butts are open at the top for easy access when watering. They get colonised by insects, notably gnats, which become a nuisance around the garden. Lids and covers have not been very successful. Is there a suitable organic sterilising agent which will contol the insects without harming the plants in the garden? The water is primarily used on fruit and vegetables.

Public Comments

1. I think it's neem oil or teatree oil.
2. a layer of cooking oil kills the gnats- if it is feasible to suck the water from under the oil layer when watering. otherwise only covers (like made of cloth or very very fine screen) can help. anything that would require something dissolved in all the water would be costly, ineffective, toxic etc.
3. Mosquitos and Dragon flies lay their eggs in still water. Acquatic larval stages must have water to survive. You don't need chemicals to kill the little "sqigglers" they will die without their wet environment. So, when you use the rain water on your garden, the larva will die, fertilize your garden in small quantities, and you have effectively reduced the future breeding population. Beautiful! Best of all, no chems. Don't you just love dirt?!
4. dish soap , it should float on the surface and make it impossible for the bugs to maintain surface tension or breathe through it, get organic dishwashing soap from a healthfood store.