You folks do know that there is some actual science on this issue?
I was curious and went looking for research papers. Here is what I found in general. The scientists seem to consider normal temperature for them to be about 82.5F. However, the more interesting part is where the experiments investigated cold tolerance. At any rate, here is the proof some in this thread asked to see. Here are just three of the multiple studies out there.
In one study they first acclimated discus for 30 days at 28C (82.5F). Then, by 1 dg/hour, they took them from 28 to 14C (57.2F), then kept them there for 12 hours and then returned them to 28C at the same 1dg/hour rate. At 6 hours and at 12 some fish were removed and killed for testing. Once the rest were back to 28C, they too were killed for testing. But this does indicate discus can survive for some time at colder temps.
Sensitivity in the antioxidant system of discus fish (Symphysodon spp.) to cold temperature: evidence for species-specific cold resistance
Here is part of the abstract from a study entitled:
Physiological responses to cold stress in the gills of discus fish (Symphysodon aequifasciatus) revealed by conventional biochemical assays and GC-TOF-MS metabolomics
Discus fish (Symphysodon aequifasciatus) is a cichlid that is among themost popular fish forwarm-water aquaria and also frequently used as the model animal for environmental science. However, little is known about the responses of S. aequifasciatus to low temperatures caused by environmental variation. Here, by using conventional biochemical assays and gas chromatography time-of-flight mass spectrometry metabolomics, we investigated the physiological responses of S. aequifasciatus gills exposed for 30 days to two temperature regimes: 28 °C and 20 °C.............
Overall, the results of this study suggest that low temperature can activate a protective antioxidant defence response and modify the metabolic pathways in gills of S. aequifasciatus, providing insights into the physiological regulation in response to cold stress in this tropical fish.
Discus at 28C and 20C
Another study compared discus kept at 28C (82.5F) and 24C (75.2F) :
Plasticity of energy reserves and metabolic performance of discus fish ( Symphysodon aequifasciatus ) exposed to low-temperature stress
Abstract
A 28-day experiment was conducted to investigate the temporal variations in cellular energy allocation (CEA), oxygen consumption rate (OCR) and activities of energy metabolism related enzymes in discus fish (Symphysodon aequifasciatus) exposed to decreased water temperatures. The steady temperature decrease (1 °C/day) from control (28 °C) to low temperatures (24 or 20 °C) significantly reduced the initial energy reserves (mainly lipids and carbohydrates) while significantly increased the energy consumption (respiratory electron transport system (ETS) activity) in the muscle tissue. ............. During cold acclimation, however, activities of hepatic alkaline phosphatase (ALP), lactate dehydrogenase (LDH), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were generally increased with decreased temperatures, indicating a compensation for the decreased metabolic rate............. Moreover, after cold acclimation the fish exposed to 24 °C showed comparable growth rates with those exposed to 28 °C. These findings reveal that S. aequifasciatus could recover from the initial energy disturbances resulted from decreased temperatures. The enhanced metabolic activity in response to 24 °C was well within the homeostatic range. It might be promising to culture the juvenile S. aequifasciatus at water temperature as low as 24 °C.
discus exposed to low-temperature stress
The experiments I read were mostly, but not exclusively performed on wild fish. However, it should also be noted that in all cases the discus were first acclimated at 28C (82.5F) for some period before the start of the experiments. This appears to be what science considers the normal ambient temperature in which they usually live.
Here is what I do know. There are wild discus in the Rio Xingu. I could not pinpoint from where in this river they are found. But, if it is in the lower portion. i.e. from below the Belo Monte Dam, then the waters there during the dry seasons get into the low 90s F. This is the only place where zebra plecos are found as well a few other Hypancistrus plecos. I keep several of these species and have been spawning them going on 15 years. One of the tricks used to trigger zebras to spawn requires taking the tank temp up into the low 90s F over a couple of months and then then dropping it rapidly into the 70sF. This would be the same thing the Xingu discus would also experience if they live in the Big bend of the Xingu.
If you are curious about Xingu discus, this is really good read -->
Finding the Rio Xingú Golden Discus
