SteveUK said:
It depends on your water pipes mainly. I think it has something to do with hot water sitting about longer in the pipes/boiler/tank and having more copper loaded into it. If you're off a newer combi boiler it shouldn't be a problem (heats from the cold water supply on the fly.
That's what I've been told anyhow...
That is my understanding as well.
Although after saying that I do fill up from the hot tap sometimes (with an old style boiler with lots of copper bits throughout) and my amanos have never seemed even remotely perturbed by it. I just add a touch to take the edge off the water when it is cold weather. More delicate shrimp (crystal reds, etc) may not be as forgiving.
I am spending £3,500 this month

on getting my central heating overhauled, including a new combi boiler. So I plan to use hot water all the time now.