cost and space is a big issue here, for starters a good size pond will cost about £4-5000 to make, then you have to filter it, to deal with tropical needs your going to HAVE to use 1 of the hugely expensive koi filters which work out at around £3000 then you would want a big powerful pump, probable a sequence one which are about £500, i don't have clue how much heating equipment would cost lets call it £100. then depending on the size of heaters etc it's probable going to cost about £100 a month to run, plus food for the huge fish it would house!!
and don't forget the shelter for it. ££??
that is alot of money for some big fish that will eat you out of house and home!!