This is incorrect. If you have adequate biological colony established, ammonia would be turned into nitrite and thusly turned into nitrates. I don't suspect large water changes to be the fix here.
If you have to do 80% water changes on you tanks every week just to keep your tanks under control; you have one of two issues; your tanks are severely overstocked or you have inadequate biological filter media in your system/tank and it cannot keep up with your stocking.
Regarding the presence of ammonia and biological media, I would refer you to the nitrification cycle to have a better understanding of what role biological media plays in the cycle of an aquarium.
I would check for a couple things, mind you, I haven't made it through the whole thread at this point. some good questions in the two quote above.
1. Ammonia in your source water. If you have chloramines in your water, you could show ammonia as chloramines are a bond of chlorine and ammonia. Can you elaborate more on your water change method and water conditioners? What kind of water do you have? Well water or municiple water? Has anything changed in your water quality report?
2. A faulty test kit. This is possible, but with the aro death, leads me to beleive that this could potentially not be the case.
3. Inadequate bacteria colony establish. I doubt this is it as well, otherwise you would more than likely be seeing a bloom of some sort.
4. Feeding too much. Based on the pics and the setup, I want to doubt this one as well.
5. Sand bed depth. With 2" of sand, you can build pockets of anerobic bacteria. Do you stir the sand when you do your water changes to release any gases that might have built up over the course of a few months?
I'm leaning towards incoming water quality. What throws this diagnosis all out of wack is the fact that the ray is still alive but the aro died. I almost want to say that the aro death was unrelated, but that is just my speculation.
