Assorted Discus (Symphysodon sp.)

Discus have a reputation as the most demanding centerpiece fish in the freshwater hobby, and it's mostly earned — they want warm, soft, impeccably clean water and don't tolerate the swings a hardier community fish would shrug off. What makes them genuinely remarkable, though, is how they raise their young: both parents secrete a nutrient-rich mucus from their skin that newly hatched fry feed on directly, grazing off their parents' sides for the first week or two of life, a form of parental care almost unheard of among fish. Their round, laterally compressed shape and calm, deliberate movement have made them a benchmark species in the hobby for decades, the fish serious keepers work up to rather than start with.