Albino Cories seem to be very sensitive to water conditions or changes in water conditions. If your nitrate tests are reliable, the level is a little high. Was the water recently adjusted for any parameter? That could also be a possible cause. It's true that sometimes you just can't explain a death but more often, there is something that you can point to as the likely culprit.
I would do a 25% pwc
anyway just to be safe. BTW
, what are the water conditions coming out of the tap or whatever source you use?