iv herd they have anti fungal properties, and yes affects the pH kinda like drift wood..
when i used it for bettas i loss less eggs, and the bubble nests stayed sticky longer, the fry thrived in water soaked in it, i never fed live food and would repeatedly get batches with 60-80% surviving. Vs with out it i would only have at best 50% wit the same feeding and care.
I can totally see the benefits in a breeding situation, especially for wild caught fish from a soft water environment but when keeping rays the regular water changes will cause an issue as you are going to be constantly adding fresh water on a weekly basis and this will lead to a fluctuating ph which will be far more negative than the pros of using.
In my opinion it is not worth the hassle of moving ph from the native water. You can cause too much stress.