I suspect it is similar to vitamins for people -- if eating a wide variety and healthy foods, vitamin supplements aren't necessary. In fact, sometimes supplements can do more harm than good. A lot of vitamins are stereomers -- chemicals that have the exact same constituent atoms in the same places, just mirror images of one another. Naturally, most stereomeric chemicals exist in pretty equal proportions (not always, but typically). However, most of the time when stereomeric chemicals are synthesized, only one of the isomers are made -- either the "right" or the "left" one. So, if you take a supplement, you could flood your body with only one of the isomers, instead of the mix that you should get. Even if you then ate some food with the mix, there would be very few adsorption sites for the vitamin, because you flooded them with the supplement.
Also, there is evidence that too much of some vitamins can be harmful -- it is looking like we are learning that there are effects of too much Vitamin B and Vitamin D.
In the end, when a human takes a vitamin supplement, a very significant portion of the vitamins pass through the body and then is excreted. Usually, there is just too much in one pill for the body to take in at once. Our bodies are designed to continuously take in a small amount, not take in the whole flood once a day. I suspect that for fish it is similar. A flood of excess vitamins probably doesn't do too much.
