Yes, instead of 3 gallons, you'd need 7. Surely, you have a Walmart nearby where you can get 6% stuff?

You need to remember that, with algae, if you're not winning, you're losing! When it recovers from your chlorine doses, it's as if you never added them. So essentially, if you don't add enough to kill the algae, you're simply wasting what you do put in.