Actually if you look at the scientific name in the link I posted that showed all the countries goldfish are living in, you will see it is the exact same species of goldfish as the ones in our aquariums. So yes, they are literally living in the same waters in the wild as so many of our tropical fish.
I am backing up my statements with facts. Check that link. That is not some random bored kid's webpage about goldfish. That is not even a well-respected goldfish forum's article. That is a professional, scientific organization's website. That information is all factual.
Lifespan is not the only indicator of thriving. In temperate regions many animals hibernate. This will slow down all physiological functions which will stretch out their life. But with all these goldfish living in such tropical nations as Ethiopia, Iran, Iraq, Mexico, Vietnam, and Uruguay it shows over and over how they are not truly coldwater fish.
We have been told it over and over, goldfish are coldwater. I was told it and accepted it without questioning it for so long. So many people have had it work out so well when they mixed goldfish and tropical fish. For others it did not work out so well. Maybe the temperature had something to do with it when it didn't work out, maybe it had something to do with one of the MANY other factors that can kill fish or cause incompatibility.
I am not saying that everyone should stick goldfish in their tank. Any combinations need to be considered very carefully. I am simply saying that if people are thinking that they are unquestionably incompatible because of temeprature requirements that there is significant information out there to oppose this and they may want to check it out. Fishbase.org lists their temperature range from 0-41C (32-105F) (
http://fishbase.org/Summary/SpeciesSummary.php?id=271). On the same page at the bottom is a link to Countries that takes you to the same page I linked to before. You can see all the countries goldfish are living in. These are established populations which means they are breeding.
I have seen that link before about their stomach. It is small and as I described earlier it is adapted to many smaller feedings throughout the day, not one big feeding like a piscivore would have. It is a stomach. In addition, digestion takes place in more than the stomach. The rest of the digestive system is where all the nutrients are actually absorbed, not in the stomach. So even if they did not have any stomach at all and nothing you could even call a stomach, they would still be perfectly capable of digesting their food, including proteins. That has to do with the rest of their digestive system and the enzymes present, not the size of their stomach.
