A health clinic uses a solution of bleach to sterilize petri dishes in which cultures are grown. The sterilization tank contains 100 gal of a solution of $2 \%$ ordinary household bleach mixed with pure distilled water. New research indicates that the concentration of bleach should be $5 \%$ for complete sterilization. How much of the solution should be drained and replaced with bleach to increase the bleach content to the recommended level?