Note that putting it against TWO external walls is not really going to help, as your joists will only be anchored into one of them anyway.
Also you will want to make sure that your tank is aligned so that it is 'straddling' as many joists as possible, so at 90 degrees to the joists.
As per above though, to support that weight for that time I would get it checked by a structural engineer before even considering it. Parachuting fish is one thing, dropping half a tonne of water, gravel, ceiling, and flooring onto whoever happens to be below at the time will be considerably more of an issue...
Another thing for your Dad to check will be the home Buildings & Contents insurances, as some will need it declared, (obviously having 400+ Litres of water sitting there is a bigger risk for them than not having it...)
There was a really good article on this I read the toehr day, but can't find the link now
I'll post it if I find it later.