The fish tank represents wealth, and it is said that it is a cornucopia with wealth and power (water). Generally, it should be placed in the position of the god of wealth at home, not under the beam. It should not be facing the entrance and exit, and there is suffocation in the entrance and exit. Feng Shui screens should be placed.
As for the position of the god of wealth, it is generally the diagonal position on the left or right of the entrance. If you are particular, please ask the Feng Shui master to decide according to the sitting of the house, and take the prosperous side according to the purple and white stars of the eight houses.