I once bought a stripped lepriconis those little bastards ate and shredded my fishes fins. RIP ( RIPPED INTO PIECES ) by a oscar that was sick of being picked on.
An alligator gar that was 12" to put with juv cichlid in a 280 I thought there was enough hiding spots instead just a buffet for the gar, FYI the Dovii survived
my friend (shookONES) tells me all the time when we're at stores don't buy a rtc, tsn or rtcxtsn hybrid. i have owned and regretted all 3 to some extent.
Freshwater flounder. This unassuming fellow was awesome, always out in the open, unusual and in a planted tank with roselines. The kid chose it and it was only 1.5". Over the weekend this little guy nipped all the tails from my roselines and killed a few smaller tetras. My fault, impulse buy from a dad, even though I knew better. Removed and told the kid he's just good at hiding.
Brycon Dentex, when they were commonly available few years back. I had a same size goliath tigerfish that was very aggressive. Once I got the Dentex, the GATF hid and it was found floating the next day.
Buying a ton of african cichlids, introducing them..and then just to have my silver arowana eat them up, unfortunately.. and that was a long time ago :\ Was really hoping they all get along.