No, I haven't experienced any problems with this. But it was just a thought that occured to me. It seemed possible that some sort of disease from fresh supermarket bought seafood could end up in my tank. I make my own fish food with ground up squid, shrimp, and clams. I'm not sure if the freezing process would kill anything potentially harmful. I was also wondering if the commercially available fish foods made by Ocean Nutrition, etc. somehow treated their food to avoid any of these problems.