Soft shell snapping turle

Question
I have had him for 1 year now...The water,the feeding,filter have all been stabile since day 1.
5 days ago, he stopped eating, apart from a few niblles/ He also doesn't swim around,he lays at the bottom, He is also having trouble breathing.
Whay is your opinion on what might be ailing him

Answer
The first question is- is it a snapper or a soft-shell? The soft-shell has a pancake-like flat leathery shell while snappers have hard, more jagged shells.

Soft-shells are famous for being prone to fungus infections and other problems, and for being difficult turtles to care for. Here is a care sheet: http://www.austinsturtlepage.com/Care/caresheet-spiny_softshell.htm

If it is a snapper, the cares should be easier- but they can also be sensitive to poor cares or poor diet. Here is a care sheet for these: http://www.austinsturtlepage.com/Care/cs-commonsnapper.htm