Firstly welcome
I would have the water tested to see what you have as far as water quality. Bullhead have a square tail the others have a V shaped tail. What kind of visibility do you have? Some folks use a fish trap to remove bullheads. As far as otters go they are your enemy if you want fish and plan on having any size to them . A family of them can and will empty a pond of fish.. just saying....I will defer to smarter folks to answer more of your questions