That, there, is the problem. The primary function of a science-fiction species in a book for film may be to be alien and unknowable; but the primary function of an RPG game's races, though, is to be playable.
Absolutely, and that is a sticky wicket. That other forum is about writing, where the limits are imagination, so I found the experience immensely frustrating. And it's all fun and games writing about intelligent dust clouds an AU across that reproduce only when dispersed by a star going nova, but playing one...
Fortunately, even anthropomorphized (or "uplifted") species can vary enough in their biologies that unusual reproductive traits, habits, or needs can offer a nice choice in role-playing opps and race design. Say, a species that evolved from egg-layers where they never directly mated. Or something like salmon or cod where their "lusts" are impersonal and manifest as a desire to return to their birthplace to spawn (an intense locophilia). Or like cuckoos where they reproduce but leave other species to rear their young. Or a race that reproduces asexually through spores that incubate in other hosts (they'd better not sneeze next to my PC!), etc. The way these beings perceive family, interpersonal relations, community, even government and religion would be a lot of fun to explore.
I admit a bias toward the "sci" in my sci-fi, so I look for game options to create aliens that, even if they must be human-ish enough to play and relate, still have odd psychologies and different motivations from us. I tend away from options that are essentially humans defined by an exaggerated trait or attitude, such as Klingons and Vulcans, or biologic structures best described as human-but-better-at-something (taller, 2 heads, 4 arms, etc.). They can be awesome for role-playing and are excellent sources for philosophizing about what it means to be human. But, I don't know, I guess I just like really weird stuff.
Morrus, in your initial suggestion of a piscine race, what occurred to me almost immediately was that reproduction may be more fishy (less family-oriented? Do the parents even meet? Are the young kept in a parent's mouth for a while?), their expectation of motion is much more 3D than ours (they normally expect danger from any direction, they feel frustrated by land-based movement, etc.), and their senses will be different (smell may be more acute, hearing biased toward lower wavelengths, they may have an EM sense, they'd need filters and special lenses to see outside their liquid, etc.) And what might these differences do to their society, culture, art, philosophy, etc.? I'd be keen on a sourcebook that made them a complex, robust PC option. Power to the
sea-monkeys!