I think that we should accept that MA3 is a
VERY wide tank, and if we want to see the whole of it on one screen, then we have to have a very much reduced-height window. - If a version with black at the top and bottom, (a la MA2.6), becomes available, then fine for those who do not like making the necessary shape window. - But to place all 30 fish into 1/3 of the width of the full size tank is ridiculous, IMO.
I quite like seeing the population change slightly as some fish swim off screen, - and others swim back on! - it all seems quite natural to me.
..... But I also have two 5x4 monitors, - so I suppose the answer is, - if you want more fish on view, then get a bigger tank!