People keep saying "one" and that is making me cringe. Last I checked, they were pretty social - I always see them in huge groups when I'm out at the lake. I'd suggest a pair at least, with plenty of space as they're large, dirty, and destructive - kinda like goldfish. However, I think catching wild fish and trying to keep them in a tank would be a bad idea anyways, as it is going to create a lot of undue stress for the fish, especially since you just impaled them through the face with a hook. Being under that stress in a strange new environment will make them prone to disease and infection, and I could see a nice puncture wound causing sommething nasty under those conditions.
But, since people are seldom deterred by the ethical issues of wild caught fish, seeing as most tropicals and all marines are non-domesticated and wild caught, I would advise that you read up on your state/country's laws regarding the removal of live fish from waterways, as it is illegal in many places. The DEC keeps quite the stranglehold on any animal 'resource' that earns them money - fish, deer, and anything you can hunt or fish - and keeping wild animals of any sort frequently results in a hefty fine if you're caught. I know that in my state, keeping a squirrel for example - even if you are a liscened wildlife rehabber - earns you a $200 fine and misdemeanor if you don't have a federal exhibition liscence.
edit: Tetra-man, before you start catching something as difficult to care for as a mature sunny, you should start caring properly for the fish you presently own. A goldfish in a less than 1g bowl? A guppy being kept completely solo? Sounds like the fish you allready have could use better husbandry. That goldfish is going to die very, very young if you don't upgrade to at least a 10g tank for now, and hopefully a larger tank with companions later in life. You should check out the goldfish care info on this forum.