Not a fish, or even a creature, but I have regretted a duckweed purchase before. In some of my "closed ecosystem"(ish) setups where they are appropriately being controlled via being eaten it works out well. But I have bought it and put it into tanks where that was not properly balanced and it shortly works out to an "... oh ... oh dear, that was regrettable."
Only fish I had that was a little bit regrettable was a velvet swordtail. He was beautiful but he just kept getting bigger until one day he reached a size where he decided he would devote every moment of the rest of his life to terrorizing everything in my community tank, and I had no other space to move him to, and was far too young and poor at the time to fix that. If I just had a new appropriate home for him it would have been fine.
"And kind of a half regret, clown loaches."
Yah, I love that fish, but they just keep growing. Similar story when I bought one when I was young and didn't realize how much bigger it was eventually going to get ... and only slowly but surly requiring rehousing all along the way at each stage. Great personality and coloration though.
"You've obviously never had MTS, lol"
So much MTS hate here
I love those guys and have never regretted buying them. If they are multiplying like wild in your tank it means there is an imbalance in something somewhere and they are feeding on it. When things balance out they will drop down to normal numbers. Or you can get assassin snails, or snail leaches, those clear out your MTS if you don't want them. If you go buy fancy MTS you can have pretty looking ones that will squeeze out all the un-interesting and ugly brown/black MTS, bladder or other "pest" snails. In one or two tanks I have that seem well balanced and perfectly fed I don't even see the MTS at all. The few in there sit under the substrate until lights-off and then they pop out of the substrate and do a little after-hours "roomba work" before disappearing again.
"A channel catfish ... " wow, lordy. That is like one step down from putting a great white shark in a large Jacuzzi. Was he at least interesting as a pet before he filled the container completely?