I've had a quick look through this thread, and the thing that stands out to be is your tanks KH is very low. I maintain both my GH and KH at 9-10.
KH - carbonate hardness/temporary hardness - is the buffering capacity if your water, and is your waters capacity to resist changes in pH. That is, water with low buffering capacity (low KH) are much more susceptible to large scale swings in pH due to other conditions in your aquarium. This is often a water parameter that is overlooked by Ray keepers (and other aquarium keepers). There is also an additional benefit of raising KH, it has a positive effect on your biological filtration, particularly for people running bead filters/K media. At roughly a KH of roughly 8.0 + your biological filtration will be 'supercharged'.
You said you don't test your pH (I do on the very rare occasion), but I have my KH buffered very high with Sodium Bicarbonate. The potential for my pH to crash is extremely low. By dosing KH with Sodium Bicarbonate it will also increase your pH to a maximum of 8.3. So, before doing with Sodium Bicarbonate it would be extremely important to test you pH to ensure that it is not low and you aren't bringing it up very fast, rather do it over a period of a couple weeks in very small increments.
Without ever testing your pH and maintaining a low temporary hardness, you really do open yourself up to sudden changes in pH within your aquarium. Like people have said, the actual pH is generally unimportant especially with multi generation captive animals, a stable pH is far more critical, and thus why KH is such an important factor. Like I said my pH is around 8.3, far higher than most people would recommend with rays, but my rays thrive, grow fast, and reproduce readily. This is largely attributed to a high KH/GH which maintain stable conditions and promote beneficial bacteria within my biological filtration and other aquarium surfaces,
It is always amazing at how often KH/GH is overlooked.
