I ordered a pound of sodium ascorbate. That should last me for years and set me back all of about $20 including shipping.

BTW- Sounds like the workshop you went to was done my a friend of mine Sheri, AKA Tilapia Mama. Am I right? She an I got into tilapia about the same time and got to know each other through Mike Sipe. I've invited her to come & join here, but she hasn't yet

I asked her how it was legal for her to have RMW tilapia. Her legal answer is that in a recirculating aquarium setup they are pets and so not regulated. If she was to sell the fish commercially it would be different. For personal use, you are probably fine, but if you are thinking of selling fish for eating, you should find out the rules for your self.