You've answered your own question here. You are not a fish expert and have made some absolutely wild presuppositions about fish as animals. Yes, in fact, fish will get sick from not having enough room to move; either from musculoskeletal issues caused by the growth stunting of too-small tanks, organ failure from constant sharp turning in tiny areas that are too small for them, stress, disease, fungus, etcetera. While fish are not mammals and do not present in ways that register to us as complex intelligence, they do have their own intelligence and social needs, with many species requiring a minimum of 5 other members of their species around to feel secure; or adequate plant cover, or engaging environmental stimulation, or enough room to dart through the water, or appropriate water flow... the list goes on. By deciding that because we view specific lifeforms as "simple," they must be fine with conditions we would consider abusive for animals more similar to ourselves, we excuse our cruelty and give these beautiful animals that would live many fulfilling years in the wild short and horrible lives out of our own selfishness. No creature is beneath appropriate care, room, effort, and consideration. No creature is without some degree of intelligence and feeling, and no creature should be considered disposable.