In college, I scrapped up all the money I had and bought a 5 gallon hexagon tank with an undergravel airstone filter. I didn't have enough money for a heater, and guess what I stocked the tank with? 3 baby oscars. The fish lasted for a miserable 3 months and succumed to ich. It's been over 12 years and I still think about those oscars... the guilt follows me to this day.
This is another story, but of someone I had met, who wanted to share with me their fish story. I gathered from what she shared that she owned a small (20-30 gal) freshwater community set up with tetras, gouramis, etc... that's cool. Then she started to talk about how her boyfriend wanted to get a "nemo-fish", so I was anticipating her to share a saltwater tank experience. This is the gyst of what followed in her words.
"So we went to petco and bought 2 nemos. We brought them home and floated the bag in the tank to even out the temp. But when we put the fish in the tank, they started to spaz out and in 20 minutes they were dead."
WTF! They put the clownfish in their freshwater tank! What appalled me even more was when she said that she went back and bought another one to try again. Of course... same result. I drop-kicked her right then and there.
Lesson is, don't put saltwater fish in a freshwater system... not like y'all MFK'ers need this advice.