They are relatively popular legal pets in Europe - though not sold in many mainstream pet shops. 100% of those kept as pets here are bred in captivity.
Having kept them before, they are genuinely about as hard to care for as goldfish but need bigger tanks and a little bit more cleaning.
Also super easy to breed, we let the spawn hatch once and ended up with about 70 larvae, they cannibalize quickly but 6 grew to full size and we sold them on very easily.
Also note most goldfish are abused, they need huge tanks 40G+ if I remember right, and not the 8-16 fl oz bowls they're stereotypically kept in. And they need filtration and would do better with a water heater. So idk if pointing to goldfish is the best indicator, even if imo you're technically correct - the refrigerating system isn't really a lot to maintain and that's the main difference.
To add to this, ideal temperature is 16C to 18C (we use a Pi to keep a running graph of the temperature). Lower is generally ok, apparently if it gets into the mid-20's they get so stress they try and escape the tank. We have a chiller, though we have also used the fairly common trick of putting ice bottles in the tank as needed (milk containers filled with water and left in the freeze for 24 hours work well).
Indoor water temp was fine for us, maybe depends a lot on the climate where you live, cool is easy here!
Hopefully we kept them well. We had them for years and in that time they spawned regularly, ended up giving our original pair up for adoption when we moved house.
Tank was smaller than 40g. You're right though bigger is better, that's the general advice with all pets (including goldfish which can grow huge!).
