I'd like to comment on a couple points mentioned, if I may be permitted.
I don't agree on, that constant temperatures are benificial for fish. Quite the contrary. Fish which are subject to a reasonable (!) temp fluctuation are more healthy. Also in nature you have sun and shade, incoming currents, differences between top and bottom, etc.
I didn't mean to suggest that "constant temperature" is advisable. I stated that water temperature in the tropical areas native to our aquarium fish is "fairly constant," and I went on to mention they vary by a few degrees diurnally and seasonally. There is a lot of data showing this.
Some advocate two sets of heaters on timers; one with a higher temperature that is on during daylight only, and the other with a lower temperature setting that is on during darkness. I've not gone this far, but there is certainly nothing problematical with the concept. I suppose I do have something like this in summer, when the warmer days raise the tank temperature (even with an air conditioner in the fish room) and it then cools overnight. The variation is about 3-4 degrees F.
Stratification in temperature will be present in most of our tanks, unless the flow from the filter (or other devices) is so strong it creates a permanent mix. Even in my smaller tanks, when I place my hand in the water to plant or something, I can feel a decided cooling as I approach the substrate. This is normal, and beneficial. The cooler water being more dense will flow into the substrate, where it will be warmed by the biological action of breaking down the organics in the substrate/water, and then rise up into the tank.
Second, fish don't work harder at warmer temperatures. Only their metabolism is faster. There are reasons why many fish breed during high temp periods. Also for raising fish brod higher temps are adviseable, because of the faster metabolism they will grow faster. The down side is that fish kept at (too) high temps for long time will also age faster.
I believe there is more to this. By "work harder" I was using a phrase from an article by Laura Muha in TFH some years back, in her then-monthly column "The Skeptical Fishkeeper." She was pointing out the importance of water parameters (being GH, KH, pH and temperature) to the proper functioning of the fish's internal processes. When the parameters stry from those for which the fish was "built" over evolution, the fish "works harder" in the same way that a car being driven up an incline must use more energy to maintain the same speed as on level ground.
Excerpts from the book
The Manual of Fish Health:
Fish have evolved in an environment where the temperature remains relatively stable and any changes that do occur do so slowly. Water density varies with temperature. In any one environment, fish acclimatize to a relatively narrow temperature range. If the temperature moves outside this range for a continuous period or changes rapidly within the accepted range then it causes stress to the fish. Although this is true as a general statement, fish species do vary in their ability to withstand temperature change.
We shall be looking at the physiological implications of temperature change in more detail later on, but in general the major effects on fish are: an alteration in metabolic rate, a disturbance of respiration, a blood pH imbalance, and a breakdown in osmoregulation function. Sudden temperature changes often cause swimbladder problems.
The physiological effects of temperature increase and decrease are slightly different: in general, fish adjust better to an upward rather than a downward temperature change of the same magnitude.
Increasing temperature results firstly in a vicious spiral of increasing metabolic rate, and therefore a growing demand for oxygen, in the face of a falling oxygen content of the water. Physiologically, the oxygen deficit resulting from high temperatures causes an increased production of adrenalin and a faster heart rate. The problem is exacerbated by a parallel reduction in the blood's oxygen transporting capacity.
High temperatures cause denaturing of body proteins and enzymes, and cells damaged by this process produce toxic matabolites. High temperature also raises the inherent toxicity of certain substances, most notably heavy metals and ammonia. And osmoregulatory problems occur at high temperatures because lipids change state in cell membranes, causing an increase in the permeability of the cells. This is especially crucial in the gills.
Byron.
