Just to clarify and cohese the information for those who don't have the book.
The Perthshire known as Fish is also known as 1980 I, and is the ninth paperweight made by Perthshire in its annual collection for 1980. These weights are lettered 1980 A, 1980 B, 1980 C etc.
This weight is described as Fish. "Three tropical fish swimming amidst blue-green seaweed over a seabed. One top facet and twelve side facets. Fish colors and seabed contents and colors vary. Signed with a P1980 cane in the base. Edition size 400/400 made."
The Perthshire known as Aquarium is also known as 1981 C, and is the third of eight paperweights made by Perthshire in its annual collection for 1981. These weights are lettered 1981 A, 1981 B, 1981 C etc.
This specific weight is described as Aquarium. "A seahorse, a crab, a snail and two fish admidst pink seaweed over a grey sand ground. One top facet and twenty-four side facets. Animal colors vary. Signed with a P1981 cane in the base. Edition size 350/237 made.
Now, a comment. If you've got one dated P1979, then you've got a lucky find and probably a test piece. It's also possible that the run was to begin in 1979 and a decision was made not to. Maybe the powers-that-be wanted the design amplified.
And, alas, it's also possible that it is not Perthshire. Chances are it may be an unusual or prototype Perthshire, but the experts here on the GMB and a lot of living Perthshire workers can get a look at it if you'll post some images of your new acquisition. This will help clear up any mysteries.