Goldfish and other carp are frequently mistreated by amateur home aquaium keepers since most of the "pretty" freshwater fish are tropical and need fairly warm water while all of the carps (goldfish & koi, etc) require significantly colder water to do well.

The "tropicals" that need warmer water are mostly highly susceptible to a parasite called "ich," (ichtyophtherius or something like that) if subjected to sudden drops in temperature, and it's difficult to avoid or to eradicate without raising temps to something around 75F (24C) or even a little higher; although the increase in water temperature is, alone without additional medications, a more reliable cure than most of the medicines. (The larval stage of the parasite reportedly can't survive temps above 75 or 80F, so they die off fairly rapidly at high enough temps.)

Water at that temperature though is extremely stressful to the carp family fish, and since in large tanks goldfish can get rather large, when the goldfish dies its rapid decay depresses the oxygen content enough to kill everything else if its not removed promptly.

It's not certain whether higher water temps result in enough change in oxygenation of the water to stress the goldfish, or if the higher temp simply speeds up their metabolism beyond what their gill capacity can accomodate.

The native carp in waters in my area appear to do well enough in lakes where the mean water temperatures occasonally get to a little above 60F, but do best when they have access to flowing water (likely a little cooler?) or where our typical wind does significant water surface turning. They don't seem to have any problem with winter temperatures, but usually have at least some open water here even in the coldest times, in the areas where they're most common.

Although they do forage on the bottom, ours are primarily vegetarian (with soft bodied water critters for dessert?) and I don't believe they often go below where there's enough sunlight penetration for at least algae to grow.

Although I've observed the local drum varieties caught in deeper channels than is common for most other local fish, they're rare enough (or are caught rarely enough) that I don't really know that much about their habits or habitats. The fact that they're in places where the water is deeper doesn't necessarily mean that they were deep in that water, and people who catch them here usually just toss them back in without (printable) comment - unless they're short on bait.

John