4.3 Patch notes re: increase in Aquarium limit:

*This update will be included in Patch 4.3, but due a critical issue, the number of aquariums will temporarily be limited to four per estate. Players will be informed when the issue has been resolved. Details are included in the Known Issues section. We thank you for your understanding and cooperation.

Known Issues:
An issue wherein placing a tabletop aquarium on top of another furnishing and then repositioning that furnishing allows the aquarium to clip through other aquariums.
* We strongly advise all players to avoid trying to replicate this issue. If an estate contains overlapping aquariums, all furnishings in the estate that are overlapping or clipping, regardless of whether they are in proximity to an aquarium, may be permanently deleted when the fix for this issue is implemented.

Dunno whether that ever got resolved yet.