Honestly, I have no idea how $1 Fish & Chips have any relation with them supporting Earth Hour but anyhow, Fish & Co. promises that all proceeds from this promotion will be donated to the WWF.
So to show your support for a good cause and at the same time, pick up a great deal, you should visit this page and spam 10 of your friends or just print out the following coupon. 
Note the very restrictive terms and conditions “Limited to one redemption per coupon, per table”.
Other than that, enjoy.