It is not recommended to place a fish tank under the beam!
The fish tank represents wealth. It is said to be a cornucopia, with wealth and power (water). Generally, it should be placed in the God of Wealth at home to collect wealth. It should not be placed under the beam; it should not be facing the entrance or exit door. If there is evil spirit at the entrance or exit door, a Feng Shui screen should be placed.
As for the position of the God of Wealth, it is usually on the diagonal to the left or right of the entrance. If you are particular about it, ask a Feng Shui master to decide according to the sitting direction of the house. According to the purple and white flying star in the eight houses, choose the prosperous side. That is.