Corydora should be kept in a group of at least 6 or more. This way they'll have far more confidence and be out and about more often.
A word of advice on the sharks though; silver/bala sharks grow to be very, very large fish and as such need huge tanks to house them in. They are also a shoaling fish and need to be kept in groups of 6 or more. I would recommend you rehome them and think of an alternative stocking.