explain why it is essential to match the temperature of the water when placing a gold fish in a new tank?

cookiecracker asked: i need a biological answer

I keep 2 koi carp, and have noticed a black sort of jelly forming in the pool of the waterfall, is this BAD?

blundetr asked: The fish appear to be quite happy, and the water is clear,blanket weed under control, there are also 3 gold fish in the pond, I hope it is,nt serious, as my koi are over 10 years old and about 14 inches long.