金魚 means goldfish and 佬 means man, typically a middle aged or older man. 的士佬 means taxi driver, an old man associated with taxis. So what is an old man that is associated with goldfish? Perhaps an old man who runs a pet fish store is a 金魚佬? The pet fish store owner would not appreciate being called a 金魚佬 because it has a sinister meaning.
In the 1960s a man in Sham Shui Po, a district of Hong Kong, would lure young kids to the rooftop of a building by telling them he wanted to show them his goldfish. He then molested them. The case was widely reported in the newspapers and ever since then paedophiles are known as 金魚佬.
I was a kid in Hong Kong in 1990s and my Dad warned me about the 金魚佬s. I somehow think his advice was a bit too specific to be useful. Not once did I meet an old man who wanted to show me his goldfish.