I agree chlorine or food grade peroxide are your options. Peroxide comes anywhere from 3% to 50% concentration. If handling high concentrations be sure to wear goggles and protect skin as peroxide burn is instant. As Northern farmer said If water has any bacteria in it, it will start to grow over time, so if it were me I would be treating every load of water with peroxide until a residual shows up on a peroxide test strip. The more bacteria in the water the more peroxide needed until a residual shows up. Once the bacteria is gone so is the smell. Chlorine will do the same thing it just depends what you want in your water. Don't let anyone tell you need x amount of chlorine, or peroxide to treat x amount of water. If it is very pure water it takes very little to have a residual and if it is very poor it takes a lot. I use dugout water and this time of year I need about 5 times the amount as needed in later spring or summer.