Here’s what Laurent Langlais, a Feng Shui master and astrologer that was featured on BBC, has to say about fish tanks and feng shui:
“It might surprise you, but I consider normal sized aquariums to be nice visual water items, but not activating much (luck).
If true Water activation is needed in one specific area – for example, a business that needs water movement to increase the flow of customers – a water fountain of a decent size work better. This is because most aquariums have very little surface contact with the open air and therefore ends up being decorative rather than true activators.
Now, a large pond with Koi fish is another story, as those animals are big enough to move the water and have plenty of contact with the Chi of the air. Thinking that a closed aquarium does the same level of activation is like thinking some (negative) Sha Chi is created internally by the corners of pieces of furniture. It is not the case, as Sha Chi needs a bigger structure and is external. The principles must account for size and location, either external or internal. Remember, the direct/indirect spirit formulas exist only externally and are to be found in naturally occurring landforms.
To summarize, I would still place the (home) aquarium in either North, East or Southeast sector as the water Element remains compatible with those sectors, but wouldn’t count on it only if a constant activation is needed. It reinforces the Water symbolic but does not move the Chi around much.