Bleach can be bad for the water pump and seals.. Well yes. straight up
But 1/4 cup to 15 gallons of water is what we use to sanatize.. I might, in your case. go a bit higher. say 1/3 or even 1/2 per 15 gallons
And another factor is time.. 1 hour 2. hours a Week
The first.. Not so bad. the week. not so good
Then flush with a mix of water and baking soda. Use about one cup for your entire tank (Dilove in warm water and add to tank)
Then fresh water.
Give it the old SNIFF test. and last a taste test.
You might want to run the bleach mix 2 or 3 times fresh water flush in between
Be sure to pump it through EVERYTHING.