What makes fish so successful in the wild?
This is a very broad question and a little unclear, but I will give you what I think is the answer is you are looking for. Fish have many strategies that make them successful in the wild. For example, think of the movie Nemo. In that movie it shows how clown fish live among the stinging anemones which protect them from other predator fish. The clown fish have a protective coating which prevents them from being stung by the anemones. The anemone sting can be fatal to other fish. Some fish form schools which help protect them from predators because it makes it more difficult for predators to attack an individual fish. Some fish like puffers swell to two or three times their size when scared which helps to ward of predators. Other things that make fish successful are their ability to find a special niche so that they can get enough food for survival. Cleaner fish or pilot fish follow sharks and rays and eat parasites off their bodies and left over food from what the shark or ray has eaten. Angler fish have a long filament at the top of their head that acts like a lure and attracts other fish that the angler fish then eat. Some fish in Antartica go into a type of hibernation in the winter where they do not need as much food to keep them alive as duirng the summer months. Hope this is the type of thing you were looking for.
Fish have many characteristics which make them able to survive in the wild. First of all, some fish use camouflage to protect themselves from predators. On the other hand, some fish have bright and vivid colors that alarms other fish that they are poisonous and shouldn't be eaten. Other fish can even inflate themselves so it seems they are larger. Salt water fish are adapted to live in salt while freshwater fish are adapted to live in fresh water. There are also fish who are able to live in brackish water, which is a combination of both salt water and fresh water.
Fish are a very vast species and come in many different varieties. They each have very unique qualities that allow them to flourish in the wild, whatever their environment is.
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.