Are sea sands dangerous to freshwater stingrays?

Hello guys, just to share my expererince, and I am still wondering where goes wrong...

2 months before in May, I tested my tank, and found out that the PH is like 6.0, as far as I know, I should raise the PH somehow, so I add some SEA SANDS and some SMASHED CORALS (smashed to very small pieces, like kind of sand).
Three days later I found out that my rays started to "spot shift", another 2 days later, I noted that they got red belly, and breathe was really heavy and fast, they also stopped eating, I knew something was wrong.
I moved them to another clear tank without any substrate, did water change everyday and added some anti-bacteria as well. I did that for about one week, then they started to recover and eat again.
After another week, I moved them back to the original tank (sea sands and coral removed).

Do you think the sea sands and corals are the main reason of this problem? and if so, any other suggestion to the PH issue?

Here comes the recent photo, as you could see, still little "spot shift" on the right male ray, but getting better everyday now.

Oddball said:

Worse than any substrate change is a rapid shift in pH.

Rapid pH shifts or a high nitrite tank are the leading killers in sudden stress and sudden death events.

Also keep in mind that these sudden changes don't affect just your fish. Your bio-filter also can be affected, or crash, from sudden parameter changes. The bio-filter events manifest themselves in the appearance and behavioral changes in your fish community.

Click to expand...

Thanks!!! would pay more attention to this issue.