Sure. There are high levels of an enzyme called thiaminase found in live or dead goldfish, rosy reds, shellfish, etc. This enzyme destroys the thiamine or vitamin B1 inside of the predatory fish that eats them. Other feeder fish with high thiaminase levels include shiners, whitefish, salmon, anchovies, herring, sardine, etc. Some people stuff pellets inside the food, prior to feeding, to combat this.
In addition to depleting B1 in the predator that eats them, offering any food fish after freezing and then thawing destroys B1 in the food fish itself, so double whammy. The longer the food is frozen, the further depleted the B1 content will be. This translates into anorexia, weight loss and weakness in the predatory fish.
Cooking destroys that enzyme in the food fish, which stops the depletion of B1. But cooking also depletes the nutrition content of the food, so it's not a good solution. Food fish like smelt, flounder, tilapia, sunfish, guppies and mollies either lack the enzyme, or have very low levels, thus are a great source to feed. Supplementing with liquid vitamins also works well. Happy fish keeping!
