What’s your opinion on keeping fish in fishtanks?

I agree with you on how it's unnatural to take something from the wild and then force it to live in artificial conditions. Whether something unnatural is good or not is I think a separate discussion to below.

I know a little bit about fish keeping so here are some things to keep in mind. There are two main environments fish are kept in - saltwater tanks and freshwater tanks (just as there are saltwater fish and freshwater fish). Saltwater tanks are harder to maintain and much more expensive from what I've gathered (I haven't ever tried a saltwater setup). Freshwater tanks are easier to setup and are more accessible to the general population. If one is a good fish keeper, the water environment should be designed to mimic the native habitats of the fish species kept. I mention the distinction between saltwater and freshwater tanks because your post seems to focus more on oceanic environments.

You are also right in that the amount of space needed relates to the size of the community of fish being kept. The more organisms kept, the larger the tank should be. (I say organisms because people keep shrimp, frogs, snails, plants; corals, anemones, all sorts of aquatic creatures in tanks, not just fish). In practice, this has more to do with maintaining good water conditions than mimicking the vastness of the native habitats. The more animals in the tank, the more waste is produced. The more it becomes critical to have a large enough volume of water to hold stable parameters (ammonia, pH, salts, temperature, etc).

When it comes to exposure to other fish, it really depends on the fish being kept. There are fish that do feel safer in larger groups (e.g. freshwater Corydoras catfish). There are also fish that are territorial who won't tolerate others of the same species (common example, betta fish) except during mating. Fish species vary on their displays of aggression towards other creatures. Maintaining a peaceful community tank takes careful consideration.

I think if aquatic creatures could talk, they would prefer an environment that's familiar, habitable, and relatively safe. I'm thinking of corals and sea anemones that pretty much live a sedentary life and have no need for the freedom to move/explore. For actual fish, I think their health depends on whether they've been bred in captivity or captured from the wild. I think those bred in captivity would be happy enough to live the rest of their lives in an appropriate tank. Those taken from the wild, however, for me is a different matter. There are situations when it may not be fair for the wild animal to be kept in captivity; in other situations, it may be the only chance of long term survival for the animal. Anyway, the animal kingdom is vast and our human longing for freedom doesn't always neatly apply to other creatures🙂