The requirements on the aquarium are that reputation must be 3 or better, sponsors on level 5 or equivalent (sponsors II at 4 etc), and that you place it touching not one but TWO water hexes. It is likely that you missed this last requirement in some way. In my experience, due to its shape, it can be a little harder to place than the cable car. I usually find a place for it by jamming the back of the “C” against two water hexes, or sometimes I can touch it to two different lakes by using the tips of the C instead.