it depends on the Qi map of your home.
a fish tank is a "water" element & some homes may not
even need one or, at least there may be a location
that would not cause anything negative to happen.
we are affected by the Qi in the rooms that we spend
the most time in, second only to the Qi mouth of our front
doors. With that in mind, i would reccomend that you place
your tank in place that you do not spend too much time in,
such as your bedroom or home office.