01-19-2013, 02:31 PM
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The other (non-cory) fish species in your tank (Alex) haven't been mentioned, so I can't assess their temp requirements. But is there a reason for having this tank at 81F or higher? This is too warm for many (= most actually) of our tropical fish, though there are exceptions. I'll summarize below.
Temperature ranges for each species such as those in our profiles are intended to give a range in which the fish can manage, though it may not be suitable at either end but only for shorter periods. I think it is safe to say that all tropical streams vary in temperature from day to night, and also from season to season and the water cools during the rains. The fish adapted to those watercourses are able to tolerate the extremes. But when forced to live in them continually, as in a heated aquarium, they may suffer.
Most species are best in the middle of their range. The higher the temperature, the harder the fish must work to carry out its normal life functions, such as respiration, eating, fighting off disease, etc. Less oxygen is available, which affects the physiology. The blood carries less oxygen at higher temps, and this can affect the blood pH. All sorts of issues.