it depends on your plumbing setup. if you have a hot water tank supplying on demand hot water then no dont use it. it sits in the attic for days collecting dust, dead mice, spiders etc.... then goes into your boiler, through lots of copper pipes (copper is highly toxic for fish) then out the tap.
if like me however you have a combi boiler which has a cold water feed and starts up when you turn the hot tap on (takes a few seconds to warm up, then goes to 90 degrees, back to 40 then up again etc...) your fine. the water is in contact with the copper for very little time and the water is mains fed rather than from a tank in the attic.
there are some good points to cheap crappy plumbing...... namely fast, easy water changes.
i've filled from the hot tap for 18 months with no problems. if i move house and end up with a hot water tank i wont be impressed..... hope that helps