Sadly Puppy really was exterminated, humanely, much like fish caught fresh from the lake or sea, she got a quick whack to the head to end it and a mighty flush to the ocean . As far as placing her in solitary confinement, unfortunately she has harassed the 2 remaining adult fish in the tank to the point of near death. And there simply wasn't time enough to set up another tank to let her live out her days alone. Not to mention it wouldn't have been exactly humane to give her solitary. Mollies are a schooling fish and to be placed singly in a tank would have likely been worse for her than a quick and sudden death.

Aside from her harassing them physically, she was also putting herself and all the fish in the tank at great risk of death as stressed fish release a kind of fishy toxin so to speak. When tank fish are stressed out the amount of nitrate in the tank can drastically increase... In a lake or pond system there is enough water to negate such problems when you have people practicing catch and release fishing, but in a tank there's no way to filter the water enough to remove them and ultimately the aquarium dies if the stressor isn't removed.

I know it seems just awful to waste a life like that, but to avoid coming off as somewhat ummm let's say Communistic... sometimes you just have to take a life for the greater good. Even if she had been placed in solitary confinement she would have simply been stressed out from being solitary and died a slow painful death rather than her quick execution.

But it does make me wish my DH had listened to me and just gone for goldfish . They only have a memory of 3 seconds so they really can't and don't get stressed by other fish. Really if a goldfish dies, whether alone or with other goldfish, it's because of one of 3 factors, not enough air, water is too warm or they have a disease, well and I suppose you could starve them to death but if you're that neglectful you probably aren't cleaning the tank either so they're gobbling up the algae that their massive amount of waste grows.