There are a couple fish that can offer positive effects like six line wrasses for instance because they eat some parasites on corals but for the most part no fish would mean a healthier system. When you have to feed fish you put food into the water that releases nutrients before even being eaten. Then the fish eat the food and eventually release waste into the water not only from the food but also from their own bodies. That is partly why some of the more experienced reefers with thriving tanks usually have very few fish in comparison. I've always wanted to leave a tank for a year without any fish to see just how much of a difference it would make but then one trip to the LFS seems to always foil that plan. I imagine a reef would establish itself way faster without any fish introduced to it as well; less nutrients, more pods, etc.
It's kind of unfortunate since part of the attraction of a natural reef is the unbelievable amount of activity because of the thousands of fish hovering about.
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Jason
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