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

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