My sympathies. Not unexpected. BTW, other fish can sense the weakness and attack, which what was probably done by the surviving RTC.
As for the water ammonia test false positive, I don't understand why keep going on a guesswork instead of exploring it as I have been suggesting? I'd start by testing tank water versus tap water or whatever your source water is and against a more pure water like bottled or filtered.
Also, I'd heed the advice of others and up the filtration as suggested by Travis or make sure the water being drawn into your canister is much better aerated, which could amount to the same thing as adding another canister.