I have a few.
1) Yasha Goby. Bought it and the thing immediately hid. I saw it peek out once for about 5 seconds a couple weeks later, then never saw it again. At some point, the idiot swam into the tiny emergency relief holes around the return pump eyelet and got stuck in the flow diverter attached to the eyelet. The only reason I know this is because after eons of not seeing the fish, I was cleaning the tank and when I removed the flow diverter I found what was left of the carcass stuck inside. What a waste of $70. If your tank has ANY holes a fish can potentially fit in, there's a good chance it'll try.
2) Flame Hawkfish. It's fine (so far) with my clowns and cardinals, but it murdered both black mollies within the first 2 weeks they were in the tank together. The mollies were the sweetest little fish who just wanted to swim about and nibble on the rock work and sandbed. The hawk apparently thinks the sandbed and rock work are its property and will murder anything that touches either one. One Molly it bit off its entire tail down to the meat and the other, it took a gigantic bite out of its belly. Both were dead within 24 hours of the attack. I hope it doesn't decide the other fish in the tank deserve death, because if it does, this fish is getting the boot.
3) Black mollies. Not because they are bad fish, quite the opposite. It's because when I had them in one tank, the clownfish went after them incessantly and did serious damage to their tails. Then in my other tank, the hawkfish decided they were not welcome. I kinda regret converting them to saltwater -- but my guess is that if they went into my FW tank, the betta would attack them. Seems these fish are targets for literally anything bigger than them, despite them being totally harmless and inoffensive.
I didn't put clownfish on here because other than the clowns in one tank specifically attacking the mollies, the ones in both my tanks have been awesome little weirdos.
