I have found that my shower head (Water pic or equal) is attached to plumbing that is international pipe threaded (1/2" male threads). To fill an aquarium directly takes an adaptor (IPS x hose threads) and a garden hose. You could substitute some vinyl tubing and a hose repair kit to make a more flexible hose.
I have made it a little more complex so I do not have to take off the shower head each time, but the concept is there.
I know in some older houses it seems none of the plumbing will accept threaded fittings of any sort. However, a garden hose will work to fill a garbage can. This is only cold water, though. You would then have to heat it if you were doing any sort of large water change.