I got > 90% of the subscribers from SEO over the course of 2 years. The growth rate of those subscribers has dwindled ever since my sites got punished by Google

I got > 90% of the subscribers from SEO over the course of 2 years. The growth rate of those subscribers has dwindled ever since my sites got punished by Google Panda. For those curious, the newsletter has to do with a very niche product that people usually order every single month. I'm simply an affiliate and write a newsletter with tips for that product along with a coupon code. The affiliate program is pretty friendly as I get paid a commission every month even if I don't get them a new customer (but get an existing customer to order it through my link).