Goldfish

First, goldfish tend to thrive at sub-tropical temperatures, which means about 68 to 72 degrees Fahrenheit. This is not to say that 75-degree water will kill the goldfish. It's just that the lower temperatures are healthier for them. Cooler water holds more oxygen, which larger goldfish require for good health.

Second, goldfish tend to be much more massive than tropical fish. As they grow, they can easily eat more than the tropicals. The latter may end up suffering from malnutrition. At the same time, goldfish also produce far more waste products than the typical tropical fish. Some tropical species require very clean water and goldfish will make this rather difficult to maintain.

Third, temperate water fish (such as goldfish) and tropicals tend to carry different diseases and parasites and tend to be resistant to their specific diseases and parasites. Thus, the mixing of the two can expose each to health problems they are not at all capable of coping with.

The fact is that many species of fish often seem to get along well when they are young. As they mature, however, trouble can start. You might not notice problems immediately because aggressive or predatory behavior may occur at night. For example, your Chinese algae eater becomes both large and aggressive as it grows and could eventually spell serious trouble for your goldfish. Your first clue might be a dead fish, or two.