What makes fish so successful in the wild?

I am assuming you mean in opposition to a fish tank?  If so, I think the best answer has to do with biogeochemical cycles.  In other words, nature has a great way of cycling things like water, carbon/oxygen, nitrogen, and phosphorus so that they are in appropriate amounts.  In a fish tank, it is very difficult to maintain the correct levels of nitrogen and oxygen and fish die as a result.  If the cycles in nature are not interrupted by human or natural disasters, fish can survive.  If people dump too many chemicals (such as fertilizers containing Nitrogen) into water, then you will have fish die off.