Ok after posting this... I had another thought.
60L (16 gallons) is really too small for that many fancy goldfish. Now I am not a goldfish person and have never really kept them myself but doing a little research turned up
this article that recommends 100L for 2 fancy goldfish.
Im REAAAAAALLLLY stretching it here, but Im wondering.. the goldfish doc mentioned that fancy goldfish often have swim bladder problems because of their tightly packed abdominal cavity .. well I've been told that when fish are stunted from being kept in too small a tank that their insides continue to grow but their outsides don't (I have no clue if this true or just something someone made up). However, IF that last bit is true, I could see since fancy goldfish have this problem already that it could be compounded by being kept in too small of quarters. Now take that with a grain of salt, its purely conjecture on my part but I do think its something to think about it.