We always like to "acclimate" our fishes as to not stress them out. Most Cichlid tanks are warm water, so when you add its a no brainer to add water that is close to the actual tank temp, which is warm water.
If you don't agree then make a cold bath and toss your fishes in there, lol
thats for you, you live pretty close to the Equator, so it must be very warm in there, there is no need to use warm water from your water system because the water in Singapore usually warm enough for tropical fish!
Oh yah, kind of forgot that most of you live in cooler countries.
Keeping tropical fishes in SEA has its advantages. Our tap water here is about the same as the tank water. I even stop using the heater since it is off most of the time. I set to 82F but the usual temp of the water here is about 86F.
As for OP, you can add the new cold water bit by bit. Add a moderate amount and wait for the temperature to equalise before adding again.
