Yancey et al. (2014) measured trimethylamine N-oxide (TMAO) — a small osmolyte that counteracts protein-destabilizing effects of pressure — in fish across depth gradients. Concentrations rise linearly with depth.
Extrapolating, TMAO would reach physiological limits (≈400 mmol/kg) around 8,000–8,500 m. The deepest fish ever recorded — the Mariana snailfish Pseudoliparis swirei — was filmed at 8,178 m. That's precisely where the biochemistry says fish should stop.
Below ~8,500 m, the deep biosphere is dominated by invertebrates (amphipods, sea cucumbers) and microbes — organisms with different osmolyte strategies or lower metabolic complexity.