So one thing I noticed in the picture was the top of your water.. The shot with the Aro in it.
Your water has bubbles on top of it which can usually be a bad sign.. I would keep preforming small frequent water changes. Maybe add some aquarium salt.
Also, get the surface of your water moving, lots of oxygen. If the tank has lots of tankmates, and even possible amounts of wastes, your dissolved oxygen levels could be low. Add plenty of oxygen and see how the ray reacts.
If this is a consistant thing, it might not be good. Make sure you check your water paramaters often, especially for stability.
Hope that helps!
*Edit: Just noticed you said your pH was low.. It might have crashed due to acids (fish waste) in the water. If your alkalinity is low, your pH will be more susceptible to crashes like this, so you might need a product to 'buffer' your alkalinity and hold it steady. Stability and consistancy is key with the ray, make sure your tap water and tank water match up.. Do more small frequent water changes.
Miles