Despite comprising less than 1% of Earth's water, freshwater ecosystems harbor a staggering 50% of all fish species. This seemingly paradoxical phenomenon is known as the Freshwater Paradox.

Although the ocean contains over a thousand times more habitable water than freshwater, there are far more individual fish in the ocean than in lakes and rivers, but not more species.

There are many Hypothesis for this One :

Isolation - For a long time, the top candidate for a solution was the fact that isolation leads to new species. And of course, lakes and rivers are far more disconnected and isolating than the ocean. Both because they're not literally connected, but also because their courses and connections change more quickly over time, connecting and disconnecting populations of individuals over millions of years, potentially leading to the evolution of new species.

So, based on isolation, you’d expect freshwater to develop new fish species more often than the ocean, leading to more fish species in freshwater, but a recent study found that both rivers and the ocean develop new species at similar rates, bursting the bubble on the idea that isolation alone is the answer to the paradox.

A Good Environment - And isolation ISN’T the only thing that can lead to new species – so can competition, weird mating

rituals, a variety of ecological niches, and a stable environment. As it turns out, lakes have the perfect combination of all those things and pump out new species up to five times faster than rivers or the ocean. That’s it then! Lakes must simply churn out so many new fish species that they pad the freshwater numbers.

Except there’s a tiny problem with that. Even though new species arise more quickly in lakes, the current total number of species in lakes isn’t that high - there are way more fish species in rivers! The explanation of the freshwater paradox, then, may lie with something else: extinctions.

Extinction - Then, may lie with something else: extinctions. When we look at the fossil record, we see that there have been more ocean fish species than freshwater fish species throughout Earth's history, but many were lost to extinction or transitioned into rivers to survive. Meanwhile, freshwater fish in the fossil record were less affected by extinction events, which would explain why there are just as many freshwater fish species. Except, as always, it’s complicated. Fossils form more easily on the soft, undisturbed ocean floor than busy places like rivers. So, it could be that there are more fossilized ocean-fish species not because there were more ocean fish, but because more ocean fish became fossils.

So then how do we explain the “freshwater paradox”? For now, we don’t know! It  could be some combination of isolation, competition, extinction, and so on - it could  just be an odd coincidence, or it could be a natural phenomenon or process we haven’t yet  figured out. We’ll just have to wait and sea.