I would agree with eug, and I've also noted something else.
Mikrogeophagus ramirezi (
Blue Ram) is mentioned as a possible, this fish must have warm water. The profile has 80F/27C as the minimum, and the aquarium should never be lower for this species. The
Neon Tetra will not be comfortable at this warmth, since its "high" tolerance is around 77-78F and I would maintain it lower for the reason eug mentioned, on which I'll expand a bit.
Fish obviously have a tolerance range when it comes to temperature. The waters in the tropical rainforests are not a consistent temperature, but actually have a fair degree of fluctuation from day to night and then season to season. But as always, fish in nature is not the same as fish in the aquarium.
The water in a tropical stream may be warm, but the fish has the option of moving into more shaded spots which will be cooler by perhaps as much as a couple degrees, or move into faster flowing centre channel where it will be cooler, etc. These options are denied to all fish in an aquarium, where we confine them to very specific and steady temperatures. Some do suggest two sets of heaters, one set warmer for the daylight and one cooler for the dark, with timers. That's worth considering, no doubt, but most of us have healthy fish with a stable temperature so this does appear to be suitable. But this means we must be very careful with the selected temperature.
When one sets out ranges for profiles, the low and high limits are generally safe for the fish but not necessarily long-term. It is usually safer to go with the lower rather than the upper temperature, and this is due to the fish physiology. The warmer the temperature of the water, the harder the fish must work on all counts: respiration, digestion, dealing with stress (tankmates, water parameters, disease, etc). The blood carries less oxygen with each degree of temperature rise, affecting these processes and others such as the regulation of the blood pH, osmoregulation at the gill filaments, and so forth.
Most of us know what it is like doing heavy work on a hot day; you lack energy because you are using so much. The fish have the same issues, only again they are confined to the aquarium with no escape to an air conditioned room or under a shady tree.
A degree or two may not seem like much, but to a fish it is significant because the temperature in the environment will largely determine the fish's temperature and as I mentioned the many aspects of the internal equilibrium is affected.
Summarizing this, it is always safest to combine fish that have roughly overlapping preference ranges for temperature, and in the lower and mid-range.
Byron.
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