Jim made a point that right now, the best part of the Aquarium are the fish. Once the background is redone, the roles will be reversed and the lack of definition in some of the fish will become evident.
Current texture sizes in Marine Aquarium 2.6:
310 x 280 transverse (1)
Clearfin Lionfish
160 x 100 transverse (4)
Blue Ring Angel
Redfin Butterfly
Red Volitans Lionfish
Rock Beauty Angel
128 x 128 (11)
Achilles Tang
Addis Butterfly
Copperbanded Butterfly
Emperor Angel
French Angel
Forceps Angel
Powder Blue Tang
Regal Tang
Squarespot Angel
Wimplefish
Yellow Tang
128 x 64 (12)
3 Stripe Damsel
Blue Chromis
Clown Trigger
Koran Angel
Flame Angel
False Percula Clown
Picasso Trigger
Purple Tang
Queen Angel
Regal Angel
Royal Gramma
Threadfin Butterfly
Textures for 5 of the fish are mounted transversely (45°). This not only makes more use of the space, but allows for higher quality fin textures. As you can see, the Clearfin Lionfish is more than twice as detailed as the Red Volitans Lionfish and really flexes its muscle on a 1600 x 1200 display.
To upgrade them all to 256 x 256 -- transverse or otherwise -- would be quite an undertaking! Just upgrading the dozen or so 128 x 64's would go a long way.