I have a few that I don't want to deal with ever again.
First is Green Tiger Barbs. They were so finicky. I couldn't' keep them alive for nothing. It drove me crazy, every other fish was fine, the Green Tiger Barbs, not.
Another I won't do again is Neon Tetras. I had Just taken 3 new tetras out of QT, they'd been in QT for a month, I put them in the community Tank the next day all 5 tetras were dead. I did PWC daily for several days to get rid of whatever killed them. But a few days after I went back to twice a week and all the rest of the fish broke out with ick and were dead 8 hours later. Even with QT those tetras killed off my entire tank.
I Now have a Mbuna Cichlids and they are great, except for 1 of them.
The Red Zebras are serial killers, not just that, they are sneaky serial killers. I had 1 male RZC who at first was fine, then he hit adolescents.
He decided that half the 55g tank was all his and he would hide and wait for another fish to swim past. He'd lunge out, latch on and rip. dead fish. He would do this at night when we couldn't see him. I finally caught him killing one of the females early one morning. He was back at the FS the next day.
Now all my Cichlids play together and share space and not a single injury let alone death.
And last, although they are cool and beautiful, I wont own any more Sharks. They're personalities are to unpredictable. We had a great Red tail Shark for 2 years. Then for some reason my daughter decided to move the tank a 10g, with all the fish and all the water still in it. She actually made it half way through the house before the bottom fell out of the tank along with the fish and gravel and plants. Everyone lived through it except the RTS. So we got another one, the last one was so sweet, so why not.
OMG!!! We tried 4 different RTS before we gave up. Each one of them were mean and picked on every fish in the tank. We wanted one like, Bruce our first RTS. We named him Bruce after the Shark in finding Nemo. The one that lead the Shark meeting, "Fish are our friends, not food". Our Bruce got it but none of the other RTS did.