Lolly you may be right, I have worked on a few houses where that's not the case and including my own. There are "noggings" or at least there should be holding each joist together throughout the whole house though which would give a greater support to each area, so maybe that's why I haven't seen any extra supports where a bath is. Different builders use different methods though and as I said you could be right on some houses. Baths are generally in against a wall though (some are freestanding in the middle of the room I grant you) the majority are in against the wall though so it doesn't have as much weight onto the centre of the room.
Anyway, I'm off track, sorry.
If you find out if the room below your office (under where you want to put your tank) is a load bearing wall you will be able to put more weight into that room.