Hi,
You can use water from the hot tap, but the water has much lower oxygen saturation than cold water. That said, water boiled in the kettle carries the same problems. This is fine if the water is oxygenated before added to the aquarium, but that would defeat the purpose of using hot water in the first place as it takes time, and you could have just used water from the cold tap and let it warm up.
What % of the water do you change each time? Water changes of up to around 50% from the cold tap shouldn't have any great effect on the fish, even if changed immediately without being left to acclimate to room temp. Many fish actually like the flow of cool water and will play in it, and it even encourages some species to spawn.
Temperature acclimitisation, although important, is over-stressed amongst aquarists, as the temperature difference would have to be quite large to have any negative effect. Thats not to say the dangers aren't real. With a large enough temperature fluctuation, fish can die, but in my experience a fluctuation that big is rarely achieved in the aquarium so long as you are sensible about it. Many people are scared to change, say 10%, straight from the cold tap without acclimation, and i believe this fear to be unfounded.
It is good practice to match the temperature so far as is practical, but the dangers certainly seem to be exaggerated in the aquarium hobby.
I personally do 50% changes every week straight from the cold tap (i use a python) and have never had any ill effects.
Cheers
BTT