sorry for his loss ... perhaps it was part of the aging process ... if tank has been running for several years, this is quite possible, some tropical fish are "annual" fish, others may have life expectancy ranging from 1-3 years or 8-10 years or longer ... some fish live longer in aquaria, others shorter, much depends on the water quality/suitability, tank dimensions, tankmates etc etcthe fish has now passed away
then feed very lightlyWe are now left with one ghost shrimp and one horned snail.
The tank is not open topped. We fed the fish peas and bloodworm to clear any blockages and kept him quarantined in "fish hospital" but he only lasted a few days following the onset of symptoms.
Black Moors (& most fancy goldfish) can reach 8 - 10 inch when mature - not including tail! I believe they grow to 4-6 inches fairly quickly, with growth slowing thereafter.
A 30 gal tank is often suggested as the minimum tank size for 1-2 goldfish (don't quote me on this, I'm not a gold fish keeper )
It's unlikely your tank was too cold for a goldfish, they really do better at lower temperatures ... I suspect the small tank size was the greatest challenge for this fish - that he lived 4 years in a 20 litre tank suggests that you took rather good care of this fish
Sadly many fish shop employees (especially at the big box shops) have limited fish knowledge.