You over-salt the marinade
If you flavour marinade mostly with salt, the meat will absorb the liquid but leave most of the salt crystals behind. Rub the majority of seasonings and salt directly on the fillet, adding only a small portion of the salt to the marinade. Be careful not to marinate for too long, which will make the fish soggy. Mix dill, fennel, lemon, and a little bit of olive oil for a subtle yet delicious zest.