Heiko Bleher has collected data on Discus habitats for the past 40-50 years, so he knows the answer to this one...
Each species is found in waters of a specific temperature and chemistry and they have evolved to live in these conditions over millenia. This is also why they are isolated from each other, except during extreme floods.
I have collected data on the water they are found in over the past 40-50 years and used scientific equipment and taken measurements at depths of 1.5-2.5m/5-8.2’ where the Discus live.
Symphysodon discus, the Heckel Discus, lives in water with an average temperature of 28.6°C/83.5°F. The highest I measured was 31.7°C/89°F in one biotope and the lowest was 25°C/77°F. S. discus is a blackwater species.
Symphysodon aequifasciatus, the Green Discus, lived in waters with an average temperature of 28.2°C/82.7°F through 1997. Prior to 1996 the average was 27.4°C/81.3°F. The highest I measured was 31°C/87.8°F and the lowest in its biotope was 24.6°C/76.3°F. It also lives in blackwater.
Symphysodon haraldi, the Blue or Brown Discus, lives in a water average at 28.8°C/83.8°F The highest temperature I ever recorded was 32.4°C/90.3°F and the lowest was 23.5°C/74.3°F.
It is the most tolerant, as regards water parameters and temperatures, of all three species.
This item was first published in the September 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.