Maintaining very small ecosystems

Why not have a small space ecosystem with an "ocean".
you have vapor deposit from an iron astroid you can
build the shell as thick as you want..a foot? Then you
are protected from radiation, surronded by so much
water. And with that shell you could enjoy a high
pressure atmosphere. You could have a varity of good
seafood and even invite the dolphin to space with
mankind. this is sheer speculation but people have
promoted the idea that high atmosphere will increase
resperation decreasing free radicals in our cells
extending your life.