Sodium hypochlorite solution ...

Sodium hypochlorite solution is denser than water so, if you pour a small volume into a 500-L tank of static water, it will sink to the bottom and mix only very slowly through diffusion. To mix it, add the hypochlorite as the tank is filling (assuming there is some turbulence), or stir the tank manually. Adding a pump or mixer would do the job too, but is probably excessively complicated for just a 500-L tank. It is best not to add the hypochlorite before filling the tank in case it corrodes the tank, splashes, or damages the tank through exothermic reaction when diluted.