Honestly I can only speak for my own but there was clearly a difference in behavior when I added a tankmate for him. Much more lively, active, "happy". With 3 they always stay together rather than apart. This would be 15g a fish scenario and could work but only if large weekly water changes greater than 50% were done. Just imho. Or could also be done if Goldfish would be willing to upgrade his tank again in a year or so. I'd rather see two than one. Sure you can't do that 160l? lol
I would love to get two or more goldfish to make sure they were happy, but I don't know whether the 160 litre would be okay or not. You see, it hangs over the table where I was putting it by about 5cm on each side.