Losing a fish always sucks ass. I've been there a few times myself.
PH levels drops once it's stablalizes and it can cause stress to your fish if there's a constant ph swing. To avoid this just add some coral to your tank to keep it from swimming.
If you decide to get another ray try a slow drip method to get her acclimated to your tanks water parameters, and practice proper water management since you know high ammonia is coming from your tap. Is AquaScience available in your area? It says it "removes" ammonia instead of binding it to make it less harmful. I use prime to treat my age water (just a habit), and use aquascience when I w/c. I do 25% every 3 days religiously because I have 10 Rays in a 833 and I feed twice a day. You can imagine how high the bio is (these are pups so there's plenty of room for now)
Let me know before you get another one and we'll make sure everything is perfect before introducing her to your tank