2 ways of thinking about this, and so far everyone has taken the side of the poor fish keeper.
Now, don't get me wrong I love Koi carp and keep them myself in an 8000 gallon pond. I know how much they cost and how much the equipment, food and more importantly how much electric it takes to run a pond of this size.
Now, for the other side of the argument..... it's not his house is it
It belongs to his local housing association. How would any one think if they decided that the best cause of action when moving to the Philippines was to rent their property out in the UK, only to find that when you came home your tenant had built a effing great big pond in your garden. Did you declare this when you took out your landlords insurance?
As I said, two sides to every story