the st. johns river rays are a group of atlantics that stay in the river system for life, vs the atlantics you find in the ocean.
now, if you had the power to replicate a water source the size and flow of the st. johns river, of course you wouldn't have to worry about their urea output. would any of us ever be able to achieve that on our own? no.
there is such a vast volume of water in a river system that is constantly replenished, not to mention constantly being filtered by bacteria and plants on a massive scale that it keeps itself clean.
we simply can't achieve that in a home aquarium. brackish/salt water lowers their output of urea sizeably, so in home aquaria, it is not only the easier solution for maintenance, but also for the health of the fish.
that is why most people who buy atlantic rays can't even reach the two year mark.
