am i missing something, i think the OP may be buying a second hand tank complete with fish, if this is the case then you are best to transport the fish and as much of the water as possible from the tank, fill it up, plug all the equipment in so the tank gets up to temperature and then add the fish to the water.
normally we'd say use all new water however when buying second hand tanks from places like ebay they are often being sold on becfause people don't have time or want to care fro them anymore, therefore they are often poorly maintained and have old tank syndrome.
tanks with old tank syndrome must be treated with care, a large water change will likely kill the inhabitants so you need to do small water changes very frequently.
there's a pinned topic in the beginners resource centre which you should read on the subject of old tank sydrome.
do you have a test kit, if not you need to make this top priority, really you need it before the tank comes. you need a liquid based test kit, i recommend the API Fresh Water Master Test Kit, plenty of fish shops stock it, if no one near you has it then ebay always have them for around 15/20 quid.
you'll need to test the water as soon as you get the tank in and set up, if you post the results up here then we will eb able to take a rough guess at the health of the tank and the liklihood of old tank syndrome and advise accordingly.
of course if i've mis interpreted and it's a second hand tank but not coming with any fish then BTT is quite correct
