Actually, that's not necessarily true. In some cases excess vitamins can lead to hypervitaminosis, a condition which over time can lead to toxicity. This is especially true with fat-soluble vitamins due to their ability to accumulate in the body, hence they generally have a higher potential for toxicity than do water soluble vitamins.
That, and one is pretty much watching their fish piss good money down the drain if one is already feeding a high quality pellet. If feeding fresh/frozen as the staple that's a different story.