Didja ever measure the ammonia? Depending on the rock's makeup, you may have a lot or little to no ammonia. I see you're adding food, but ya gotta know where you are on the ammonia to guage what you're doing.
Also, since decay and ammonia is slower to occur in colder temps, I'd consider keeping the temp nearer to the 70s, even the higher 70s. 59 to 75 degrees? Need a thermometer too, but I would get way above 59.
"Life may not be the party we hoped for but while we are here we might as well dance!"