It may be off topic for the OP's thread, but as another member has asked about goldfish and tropicals and this was in the OP's initial post, I will respond from my understanding. Let me begin by repeating that I am not experienced with goldfish, but my research over many years has clearly made some distinctions between goldfish and tropical fish.
First I must ask that you give
serious consideration to the point made in the blue citation in my signature block. The fact that any one of us tries this or that does not mean it is wise, and it most assuredly does not mean that the fish are not being detrimentally affected just because they don't turn belly up. None of us can possibly know what is occurring in the fish's mind or physiology when we step outside the norm as the fish expects it to be. Each species has in their DNA the inherent print for life, and it is reckless thinking to assume we can ignore all this without some repercusions.
No reliable source has ever, to my knowledge, recommended combining goldfish and tropical fish, and quite the opposite. There are several reasons. Temperature differences, food differences (goldfish and tropical fish having differing food requirements), temperament differences, different disease tolerances, and compatibility issues. I will deal primarily with temperature, since it was specifically mentioned in post #6 and there are clear midunderstandings.
Water temperature drives a fish's metabolism, so it is extremely important. Goldfish are sub tropical temperature fish (which may be a better term than "cold water"), and their physiology operates best in such temperatures. I will cite some passages on the effects on all fish of increasing temperature, taken from
The Manual of Fish Health authored by four of the most qualified ichthyologists in the field, since I couldn't possibly say it better.
Fish are ectothermic, which means that they have little ability to physiologically maintain a constant body temperature; in most fish it is usually the same as the temperature of the surrounding water. In general, fish species are loosely divided in terms of temperature range into warmwater species from the tropics that live at temperatures over 25C (75F) and coldwater species from high latitudes where seasonal averages are well below that level. ...
As we have seen, it is important to maintain the temperature of the water within the natural range of the species being kept as stable as possible. ...
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 (since rising temperature reduces the quantity of dissolved oxygen that water can hold in solution). 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. Behavioural changes noted in fish during high temperature thermal stress include heightened activity, a loss of equilibrium and increasing ventilation rates.
High temperatures cause denaturing of body proteins and enzymes, and cells damaged by this process produce toxic metabolites. High temperatures also raise 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.
Under extreme thermal stress, a fish slips into a coma as the central nervous system shuts down. The maximum temperature a fish can survive depends on the species, the temperature to which it is normally acclimatized, the amount of dissolved oxygen in the water and the level of toxins present.
The temperature of 27/28 C mentioned in post #6 is not only too high for goldfish (so far as I am aware, I will accept correction from a reliable source), it is even too high for most species of
Corydoras. There are some that can manage very well at this warmth, but not all of them. Again, they probably won't die rapidly (though some might), but long-term their physiology
is being affected and aside from stress this causes serious health issues and inevitably a less than normal life span.
Byron.