After I quit laughing for a couple of minutes, with some very funny mental images, I have two suggestions:
(1) sell nothing but Zebrafish
(2) make up a loose leaf binder, indexed for major types, with pages in each section containing lists of the various fish, birds or whatever. Each item could have an image, enough of a description to to pick out the right item, and a bar code. The bar codes can either be printed on labels and pasted on the page, or there are fonts available for word processors that will print the bar code for whatever text you type in.
You can be as detailed as you like. If you don't need to track each individual species, it could be as simple as "large fish", "medium fish" and "small fish", or "premium fish" and "guppies". Or, you can include each type of fish you sell, if you use the program to re-order by type, or if they have widely varying prices.
That's the way we handled brass fittings. It's also the way we handled items that were too big to get to the register. It was a lot easier to pick out the image of a red, black, blue or green Weber grill in the book, and scan the code, than it was to scroll through all the listings for Weber grills in the computer.