I often advise people how I do water changes, and this is obviously depends how big your tank is.

If you have a large tank like mine (1000ltrs), I will get the hose from outside, stick it in my tank, turn on the outside tap, this preps the hose with water, then I unplug the hose from the tap and let the water back flow down the drain. Once down at a good level, I reconnect the tap, and turn it on till it fills the tank back up.

Yes the water I put in will be cold, and if it is freezing I only do a couple of "s a time, however most times, I do 30/40%. The two 300w heaters kick in and the rest of the water in the tank soon helps get things back up to temp.

Shelagh, can you attach more hose to this to make it longer? More to the point - did you buy this, and if so, do you think it could be extended? Pic on the site doesn't show much - reckon you could mess with it so you could fill the tank up again using this? I really need a python but they are sooooo expensive!