As a side note the upside to adding in cooler water than the tanks current temperature is an increase in the waters oxygen level (which is why I feel when you do a water change to always add in cooler water than the tanks current temp). Did you know water at 77F/25C has an oxygen solubility level of 8.6mg/L? Which means that the average oxygen level in a typical indoor freshwater aquarium is probably going to be around 8-9mg/L. Water at 32F/0C which is 'freezing point' has an O2 level of 14.6mg/L, while water at 212F/100C which is 'boiling point' has an O2 level of 0.
To break it down if I did a 50% (1/2 the tanks water) water change I would need to add in 69F water to my 79F for it to average 74F, which is a 10F temperature difference from the tanks current temp. So if I am only changing 25% (1/4 the tanks water) of the water I would need the new water to be 15F below 79F which would be 64F.
video on water density. So, if you happen to be using a tube to fill cooler water back into your aquarium you'll want this tube to be closer to the waters surface and not at the base of the tank in order to allow the two water temperatures to better merge together (unless of course you have other intentions for doing so).