Keep in mind that the focus distance is measured from the sensor plane to the subject. The distance from the front of the lens to the subject is called the working distance. You might have two lenses with the same Minimum Focus Distance that have widely varying Working Distance.
The Tamron SP AF 90mm f/2.8 Di Macro Lens (Tamron lens ID 272) has a working distance of only about 3.6 inches.
The Tamron 90mm f/2.8 Di VC USD Macro Lens (Tamron lens ID F004) has a working distance of about 5 inches.
The Tamron 90mm f/2.8 Di VC USD Macro Lens (Tamron lens ID F017) has a working distance of about 5.5 inches.
The Tamron 180mm f/3.5 Di Macro Lens (Tamron lens ID B01) has a working distance of about 9.6 inches and might be better suited to your use case.
Also keep in mind that the refractive properties of the water in the aquarium and, to a lesser extent, the glass of the walls of the aquarium will affect focus distance, working distance, and maximum magnification. In this case the water should work to your advantage.