I seriously doubt whatever it was killed adult fish. If it was a dragonfly larvae definitely not. If it was a leach, definitely not unless in huge numbers which I expect you'd have noticed. If it was a slug definitely not, because it would have drowned. If it was a hydra definitely not because they can't kill much more than fry.
Larvae of the last surviving lifeform of Ceti Alpha V. Their young enter through the ear and wrap themselves around the cerebral cortex. This has the effect of rendering the victim completely susceptible to suggestion.
If your goal is to minimize the number of animals they kill? Perhaps marginally because more would slip out of their grasp. But to be really effective you'd also have to trim their teeth.
Most of the time declawing is done more to protect furniture from them sharpening their claws. In which case trimming is again not particularly effective because many cats react to having their claws trimmed with a flurry of claw sharpening,frequently on furniture.
Current information says you'd need a tank of approximately 1,300,000,000,000,000,000,000 liters. Noone knows any way to raise the eggs other than in the ocean.
WOw, maybe if they all looked like yours, disgustipated, they'd have a better reputation. But all I've ever seen are hordes of pretty blah looking likely over inbred ones.
You sure it's CO2? Have you ever inhaled CO2? It burns and you immediately feel like you're suffocating. It's the high CO2 level in your blood, not the low oxygen level that triggers the feeling of suffocation.
Can you buy a plant that has been grown emersed and then plant it emersed? Yes, yes you can. Buying a plant that's grown submersed and then planting it emersed is a little more difficult but as long as they're kept damp and a type of plant that can do it, it should manage to convert itself to an emersed form.