What can it be treated with its only on the 1 fishThe fish looks bloated and looks like it may be developing dropsey. Are the scales sticking out? Has the water been tested?
4000 litres 12 foot long about 17 fishDropsey is caused by poor water quality. If you father in law has a test kit, have the water tested.
If you can give us some info on the pond- gallons, number of fish, size of fish, filtration etc, that would help