Optimum temp

[seangee]

Is it better to be at the lower or upper end of the recommended range for species.
This is what is in my community tank and the SF recommended ranges.

Cardinal Tetra: 23 -29
Glowlight Tetra: 24 - 28
Peppered Cory: 22 - 26
Sterbai Cory: 24 - 28
Dwarf Chain Loach: 20 - 30
Bristlenose Pleco: 21 - 26

Before summer attacked us I had the temp set at 25. During summer I turned the thermostat down to 23 which would allow the tank to cool further as we approach this time of year. Well the heatwave has finally broken and the heater kicked into life on the weekend. The peppers loved this and were far more active than I have seen them for ages. The glowlights seemed less happy and remained tightly shoaled in the upper half of the tank. I have upped it to 24 and everyone seems to be behaving normally again.

Should I leave it where it is or edge it back up to 25 so that I am not at the bottom of the range for anything? (and will it make any difference?). I only mentioned those 2 species because nobody else seemed to notice or care. (Apologies to the N. Americans - my head doesn't cope well with farenheit).

 

[Byron]

Fish are ectothermic, meaning that they do not generate internal heat but rely on the surrounding water. Since temperature drives the metabolism, this is very important; and tropical fish species function best within fairly narrow ranges. Tropical habitat waters generally do not fluctuate very much in temperature, diurnally or seasonally. A couple degrees is not very much, and that is about all the fluctuation there is in waters of South America and SE Asia.

The temperature range for a species in reliable sources (like SF) means that the fish should have no significant issues within that range, but it must be kept in mind that in most cases the fish will not do very well as either extreme long-term. Mid-range is usually the safe area for long-term maintenance, with the high and low extremes tolerated short-term such as during hot weather and similar temporary events. Again generally, fish can tolerate warmer better than colder which is why raising the temperature to treat disease is usually less problematic than would be a heater failure resulting in chilling the fish overnight, which at best may bring on ich and at worst kill the fish.

Having said all that, fish tend to function best (= with the least effort) in lower rather than higher temperatures within the range. Again there are some exceptions, but generally lower is better. The reaction of your cories is due to this, without doubt. The warmer the water, the harder the metabolism must work just to maintain normal bodily functions, and long-term this can seriously weaken the fish, leading to a weakened immune system, stress, shorter life-pan, and health issues that the fish should normally be able to fight off.

My tanks are all set for 24.5 C (76 F), though I have last week lowered this a tad further to 23-24 C. During the heat we were experiencing here this past six or so weeks, I unplugged the heaters on all tanks. I have a room air conditioner which kept the room air temp below 80F (26.5 C) so the tanks were rising to this during the day. At night, the room temp tended to lower to 74F (23 C) and i saw no reason to be heating the tanks above this only to have the warmth of the day raise it higher. This is the first year I have done this, and I must say I think the fish did respond well. It cooled off this weekend, and the room temperature got down to 70F (21 C) and didn't rise much above that during the day so the heaters are back on now.

 

[seangee]

That sounds like a similar weather pattern to ours. In the absence of aircon the best I could do in the last 6-8 weeks has been 26 overnight with the daytime tank temps regularly creeping into the 30s. The only exception to this being the few hours after my weekly water changes. I think I will leave it at 24 for the next few weeks in case we see a return of high daytime temps - although I suspect the internal temperatures are unlikely to return to the previous levels this year.

 

[AbbeysDad]

I don't know that I can really add more to what Byron wrote. Within acceptable ranges, lower temps would slow metabolisms resulting in less food requirements and slower growth, but may also result in longer life spans. Higher temperatures increase metabolisms resulting in greater food requirements and faster growth. and perhaps a reduced life span.
With some exception, I've always kept temps in the 76F to 78F range.