Not too sure about weight, it’s more about surface area. Keep in mind rays produce massive amounts of ammonia compared to other fish and are more susceptible to ammonia poisoning. This is why most ray keepers tend to go overkill on filtration, such as myself.
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Rays typically need a sump the size of 1/3 the aquarium volume. So for example your 300 gallon aquarium would need a 100 gallon sump completely filled with bio media and some form of mechanical filtration. This is the reality if you would like the rays to thrive. Be prepared for them to outgrow that 300 gallon, cheers!
It's not
more about surface area; rather, it's
all about surface area. My comment regarding weight was simply meant to point out that with a large enough volume of biomedium, you might as well find a medium that gives you the surface area without weighing hundreds of pounds, to make handling and cleaning easier. Since just about any decent biomedium will give you more than the surface area you need, things like weight, cost, etc. are really the deciding factors for me.
I don't know anything about rays, but it's too simplistic to say that for rays (or any other species) you need this size of sump or that volume of biomedia to be successful. That does not take into account the total bioload of the aquarium; rays may well produce more ammonia per pound than many other fish, but the number and size of fish in the tank is still a critical factor. Let's say a ray produces three times the ammonia, per pound of body weight, that is churned out by an arowana, for example. Don't know if that's true, I'm just making up an example. If the total biomass of rays in Tank A is only one third of the biomass of arowanas in Tank B, then those two tanks would both be properly biofiltered by an identical bacterial population...and that population will be the same whether the biomedia fills a 100-gallon sump or any other size. Enough surface area is enough; more is not better or worse, but it is unnecessary.
Maybe if you want a crowded tank...and that's a "loaded" term that means something different to each aquarist...then you really do need 100 gallons of biomedia...but just saying that rays need that much doesn't really answer any questions, and begs many others like "how many rays?", "how big are they?" and "how much are they fed?"
A friend of mine in Ontario was into goldfish in a big way, and kept most of them outdoors during the summer and indoors during the winter. His overwintering stock tanks in his basement were CROWDED (by my standards) with big fish, often 15-20 fish in the 10-12 inch size range per 300 gallon tank. The fish were maintained at around 65F for the winter months, were fed generously, and received a 100% water change about every 3 or 4 days (continuous drip). They always looked fantastic, the water was tested before each water change and although nitrates often exceeded 30ppm before a change, ammonia was never detectable. Those 300-gallon tanks were each filtered with two large foam filters, powered by two central air systems in parallel as a failsafe. The foam filters looked identical to the largest size Poret foam cartridge available today, which is 19 inches tall and 6x6 inches wide and deep. One alternating filter was removed and squeezed/rinsed every three or four days, so each filter was cleaned weekly.
Those tanks were usually completely clear, and were always completely ammonia-free, using a pair of sponge filters with a total volume of less than 1 cubic foot of foam, i.e. less than 7.5 gallons of biomedia by volume. Gee, I wonder what the water would have been like with 100 gallons of biomedium?
Surprise! That's a trick question; the water would have been exactly the same. Zero ammonia is zero ammonia.
