Rays are really simple to keep as long as you keep up water changes, understand the basics of water chemistry...cycled, adequate filtration and a varied supply of correct food.
Most rays die due to ammonia spikes caused by poor filtration, uncycled tanks or shipping damage caused by ammonia in the shipping process.
Wild caught rays that have high parasite loads that have not been properly acclimatised an treated often struggle at the hands of a novice WATER keeper.
Nitrate over time can creep up and cause problems but this is a product of success in the other areas....good fish keepers and healthy rays build nitrate to dangerous levels in 2 weeks, really good fish keepers manage the nitrate and they breed large, healthy rays.