I've got an in tank ammonia monitor that never showed any rise from 0 and I did a couple tube tests but even still I may have missed it if there was a backup in the system. I'll go talk to the guy on the corner and see if I can get some dat cocaine 
Discus do better at higher temps. 80 degrees seems very low. 86 degree should be the gosl you should maintain. I know some keep them as high as 89 degrees. I've kept Discus for years and temp is a constent 86 degrees.