well you can over filtrate to help break down the ammonia.. but you will need to do a ton of huge water changes in order to combat nitrate accumulation. The atlantic freshwater rays arent really freshwater, they just adapt to freshwater by producing more ammonia. People usually deem them sensitive and unhardy, but it's because they put them in water with low dissolved solids and they produce massive amounts of urea/ammonia and kill themselves in the process..
and the Motoro will eat all the food as they are a more aggressive eaters, and will completely stuff themselves.. so you will have to overfeed the tank in order to get the atlantic food to get enough food, which then circles back to the water quality issue..
They really do best in very large brackish/marine tanks..