Use one Akismet License Key in Multiple WordPress Blogs
Akismet is one of the best WordPress plugin available to combat spam comments on WordPress blogs,however if you have WordPress Multisite setup you may have to input the Akismet API key for every blog in the multisitenetwork,but you can avoid this by adding a piece of code in your wp-config.php file which apply the akismet API key automatically.
Add the below code in wp-config.php file:
Remember the plugin should be activated on the WordPress Multisite network and obviously change the “Akismet_API_Key” with your API key.
wp-config.php file is located in the home directory of your WordPress installation.