The use of he/she doesn't necessarily imply gender. Look at ships, especially naval vessels. They're universally referred to as "she" but that doesn't mean they're female. Another example is the almost seemingly random assignment of "der/die/das" in German. To make things worse, the use of the masculine "der" in singular form usually takes on the feminine "die" when pluralized. Your choices are limited if you don't want to give it a name. You could simply call it "Fish" and avoid the use of "the fish," use "he/she/it," or give it some sort of identifying characteristic (which would be just as awkward) such as "speckled fish," "rainbow trout," "bent fin," or the like. Ok, I'll crawl back under my rock now.