do you have any measureable ammonia after a feeding? do you have any measureable ammonia at any time period? i am wondering if you just have to many bio-balls for your system in the wet/dry. wet/dry's are generally recommended for FOWLR tanks, but they still should be matched to the system itself. as you know Elasmobranchs can not handle the nitrate levels that a boney fish can, hence your concerns. i think we need to figure out why the nitrates are rising. a decent skimmer and LR should be able to keep that bioload under control. how much LR do you have in the system. i am thinking 50# should be plenty for this. even with all of the feeding needed for the stingray. that is only two, granted large fish, in that system. i would try slowly removing the bio-balls and watch your levels closely. if you see any ammonia showing up than put more bio-balls back in. this is equalibrium for the system.
the refugium that you are suggesting will be nothing more than a settling tank. which is not a bad thing, but it will need to be cleaned out during every water change. siphon out any detritus. if not than the phosphate levels in the sump with just increase as the detritus builds up. detritus will build up because the algae in the refuge will trap it and cause it to fall to the DSB. at this point the DSB will be more than happy to absorb it. if you do not have the DSB than the detritus can be removed so that the algae will only be feeding off of the nutrients in the water column and not the nutrients coming up from the DSB. prolly a better choice for you would be a ATS (
algae turf scrubber) this is just a lighted piece of glass that water is allowed to flow over. above this plate is a large light to encourage the algae to grow. this way you are guaranteed that only the nutrients in the water column are being used up by the algae. the downsides to this are the possible yellowing of the water due to some leach back of the algae if not harvested regularly.
in order for any algae to be an efficient nutrient export it needs to be exported. if not than the nutrients are still in the system.
G~