Thanks guys !
I have tried the water dropping method and seems that to maintain the low water temperature is also a very tricky part.
I have even placed them in a smaller tank (3ft) and even then its quite a task to maintain the temperature. I might add also through my trial and error I have come across this disease called "pinecone" disease whereby the symptoms are that the goldfish bloats up suddenly with all the scales protruding out hence looking like a pinecone.
Sadly to add this is a terminal disease and true to its course, my goldfish died soon after (2days)
This disease is said to strike when the temperature of the water drops too drastically and too erratically. eg 28deg to 20deg