While all three types of fish can be kept in an aquarium, they all require different temperatures and different water conditions. Basically, saltwater fish are native to oceans and seas and so require salted water for survival. Freshwater fish are native to ponds, rivers, and streams. Many popular freshwater fish kept in fish tanks are from warm, tropical parts of the world and so require heated water to survive. Goldfish, on the other hand, are also freshwater fish but are native to much cooler water. Your goldfish will be happiest in water that’s around 65°F, while most other pet freshwater fish need temperatures of 74°F to 78°F. Because of these differences, you can’t keep saltwater fish, tropical freshwater fish, and goldfish in the same tank.