That is really sad to hear mate. It seems like many people have difficulty regarding having 0 ammonia. Personally I have never had this problem, my filtration is 2 AC 110s combined with a filstar XP3 canister filter. I have over 1000gph of filtration and my tank cycles over 6 times. That being said, I alternate every week cleaning only 1 of the filters completely. Followed by a 30% water change every sunday. Rays have a huge bioload but it sounds like that you were doing a great job with tank maintenance. Did you remove uneaten food? Did you treat your tank with any medicines that ruined biological filtration? Did you clean filters with tap water or use the water already being removed from the tank? There are so many factors that could have come into play. Im surprised how quick the ammonia killed your ray, was there any symptoms before the incident?