How much bleach do you add to 6.5 gallons of water to kill mosquitos?

Answer 1

More bleach is required the older the larvae get. Older stages of larvae require a lethal dose of 250ppm so it is best to use this concentration as a one-off treatment. CloroxTM is 5.25% w/v sodium hypochlorite so this equates to 4.76 ml of bleach and made up to 1 liter of water.

Or just under 2 tablespoons (or 24 ml) of bleach per 5 litres of water.

This can be followed up with weekly doses of 50ppm bleach (or approx. 1ml of bleach per liter).

It is better to clean the container first, wherever possible as this will also help to remove the nutirents in the water that the larvae feed on.

Reference:

BARRERA R, AMADOR M, CLARK GG (2004) THE USE OF HOUSEHOLD BLEACH TO CONTROL AEDES AEGYPTI. Journal of the American Mosquito Control Association: Vol. 20, No. 4 pp. 444-448

http://www.cababstractsplus.org/abstracts/Abstract.aspx?AcNo=20053009439

Answer 2

Need to know if you are making a spray for adults (although that would be a large quantity) or you are trying to stop them from breeding in that amount of water (say a fountain for example) It may not even be necessary. I am assuming they are present from your question but if just curious as a preventive and water aerated, they may not develop. Add details for a fuller answer.