use the 1 to 9 ratio.. 1 part bleach 9 parts water. you choose the measurement for the size tank you are cleaning
Eample say you have a 20 gal tank you may usea total of 2 cups of liquid which means

1/4 cup bleach and 1 3/4 cups water=2 cups of cleaning solutiion...

Get it?

I usually put it in a spray bottle for later use and cleanings.. hope this helps.