I started working at a water treatment plant (I need to point out it's freshwater, making raw water safe to drink. People always seem to think it's waste water) and I noticed fish coming in to the inlet screens.
The majority die, unfortunately. But some actually make it through alive and well. So I took some home.
Not knowing what I was doing I piled five Bluegill ranging in size from 3 to 5 inches into a 20 gallon tank, set in front of a window. You can guess what happened next.
Long story short, I'm lucky they are hearty as my Nitrites were at 7 ppm
(!) before I was advised on how to care for them.
Fast forward to today. You can see what I have in my signature. The one Bluegill in the 20 is there because he is so extremely aggressive. I had one fish at one half of the 40 (him) and five others at the other end. Tomorrow I will go to check on an 80-100 gallon unit. Not for overcrowding and water quality issues but because these fellows are so aggressive and territorial. They need room. Also, water quality is excellent. Bad things are at zero and the water is so clear you can't see it when the pump and filter is off.
After having these "natives" for a while now I can't see owning any other kind. (In fairness, I've never really had any other kinds) They are so much fun to feed and are afraid of almost nothing. I reach in to move a plant or something and they swim up and check me out. I try to push them aside with my hand to get to something and they don't want to move! They are like puppies; they watch me when I walk by and beg for food. I feed them often, maybe too much. But nothing is left over and water quality is perfect.
I am hooked. No pun intended.