Encore who handle the sales of the CD versions got the bright idea to create a bundle of MA2.0 and MA Time and then rename that 2.5. Not very clever IMHO.
Marine Aquarium 2.6 features the beautiful Clearfin Lionfish, with textures 4x as big as previous Marine Aquarium fish, and the Achilles Tang. There are two backgrounds, the original and the Crystal background with the cycling calendar/clock.
Looking at the Mac system requirements
here at Encore's web page I can read
"G3, 350 MHz or higher (NOTE: Not for Intel processor based Mac systems)" which suggests your iMac won't handle it. I assume the same text is available on the box as well. Depending on what the clerk at your store told you you might be able to get a refund. Otherwise I suggest downloading the trial version of 2.6 from
here and see how it works.
/Tiny Bundle