Do not keep them in freshwater and you will be fine. They produce high volumes of waste when in freshwater and just about any home aquarium will not handle the load. Keep them in heavy brackish or full salt and they will be fine.
Is this based on personal experience?
I've seen a couple of people mention this point but I'm curious to know if any aquarists who own these rays have actually done systematic measurements of tank parameters to back up this idea. I'll admit that I haven't personally housed D. sabina, but I do know a few things about their physiology, and I think this idea might reflect a misunderstanding of the somewhat complex osmoregulatory physiology of these rays. If people have actual first-hand experiences with these rays I'd really like to hear it, but most people just seem to repeating things they've heard from other people.
I'll try and provide some basic info about urea metabolism to explain why this idea of high waste production doesn't really make sense to me:
When most of us talk about "waste" production, we're referring to urea (which is what results in the ammonia that we measure in our tank water). It's important to note that there are a few steps involved in this process - there's the generation of urea, the excretion of urea and the reabsorption of urea in the kidneys.
The production of urea is based on the catabolism of amino acids. So essentially, the amount of food eaten = the amount of urea produced. For those people who've actually housed Dasyatis sabina, do they eat more than the average potamotrygon? It seems to me that the only way for these fish to really be producing much larger amounts of urea is if they're eating much larger amounts of food.
The urea that is produced by amino acid catabolism is excreted by the kidneys and rectal gland. In the case of Dasyatis Sabina and most other elasmobranchs, the kidneys also reabsorb a lot of this excreted urea. Saltwater elasmobranchs evolved this ability to maintain a higher blood/plasma osmolarity than the surrounding seawater so they don't lose body water.
Freshwater populations of D. sabina don't need to maintain as high blood osmolarity as saltwater populations, so they don't reabsorb as much urea. It turns out, that D. sabina housed in FW actually synthesize LESS urea than when in SW; however, they don't reabsorb as much, so they end up excreting more. As a result, D. sabina housed in freshwater have about half the concentration of blood urea of D. sabina in saltwater (~200mM vs ~400mM). So yes, a SW D. sabina will be releasing more urea into the water than a FW D. sabina, but this is because it's storing less, not producing more.
But here's an important point that most people don't seem to be factoring in: even though a FW D. sabina is storing less urea than a SW D. sabina, it's blood urea is still about 200 times higher than the average potamotrygon (~200mM vs ~1mM). This is because Potamotrygon, which are obligate freshwater rays (i.e. they cannot tolerate high salinity conditions) have lost the ability to reabsorb urea, so all of it gets secreted. So this doesn't mean that D. sabina are producing 200 times more urea than a potamotrygon but instead that their kidneys are REABSORBING about 200 times more than a potamotrygon, which would just excreting all that out into the tank water.
This is why it doesn't make sense to me when people say that these rays produce high amounts of waste. If they're reabsorbing so much more urea than a potamotrygon it seems like they should be releasing LESS waste not more.
I think another important point to think about is that if D. sabina has 200 times more urea circulating it's blood under healthy conditions, this probably dramatically alters its sensitivity to ammonia/nitrite/nitrate toxicity. I would guess that a D. sabina has a much higher tolerance for these nitrogenous compounds, but as far as I know no one really has any data on this and most aquarists don't discuss it.
I realize some of that was pretty technical but here's the take home point: urea production in these fish is complex and interesting but there are a bunch of unanswered questions and I don't think the frequent comments about high waste production are really backed up by evidence. I think at best, they are a major oversimplification and at worst they are completely incorrect.
If anyone actually has more data or first hand experience on this I'd be glad to hear it.
