Can I keep Goldfish with tropical fish?

When temperatures are high, nobody keeps coldwater fish. Likewise, in areas of the world where the climate means that even an unheated aquarium runs at tropical temperatures, it is often common practice to keep Fancy Goldfish in with surprisingly different tank mates such as Discus.

Given the delicate nature of some of the highly bred forms of Goldfish, we can see that a nice warm tank is a great environment for them and they will often thrive.

The big problem here is that as a fish with a metabolic rate designed to function at a much lower temperature, Goldfish are simply too messy and demanding to live with tropical species at their preferred temperature. To use an analogy, it would be like attempting to keep rabbits in a stable with a horse! A single goldfish can create a waste load that can lead to levels of pollution that tropical fish cannot tolerate and even if the water remains clear, organic waste levels can lead to fatally high nitrate levels or at the extreme, a filter clogged with solid waste. 

Another side effect of keeping coldwater fishes at tropical temperatures is the much shorter lifespan that will arise due to the fish always being kept at unnaturally high metabolic rates. By keeping your coldwater fishes cool, they will enjoy a much longer, healthier life.