Mixing goldfish and gouramis in your aquarium is challenging, but it is possible. You'll have to consider your particular gourami species, your particular goldfish breed, the temperaments of the individual fish involved, and the environment of your tank before mixing these fish.
Gouramis and Goldfish
There are many types and sizes of gouramis and goldfish, but most of the ones you'll find in pet stores max out around 5 or 6 inches long, with larger individuals up to 10 inches. Both are surface breathers and omnivores. The two fish have a lot of similarities, so it is not outrageous to think about mixing them, but these fish originated in very different environments. Their differences are enough to make keeping them together quite difficult.
Gouramis are tropical fish. Goldfish are temperate fish. This does not mean that one prefers a mai tai and the other likes tea: It means gouramis need warmer water than goldfish can stand. There is some overlap here, and it helps to know exactly which species of gourami you have. Most gouramis like their water in the mid to high 70s, which is barely within goldfish range, but some gouramis need temperatures between 78 and 82 degrees Fahrenheit. This seems close, but is just too hot for goldfish, which can be perfectly happy in water as cool as the 50s and will suffer immune breakdowns at 78 degrees or warmer.
Behaviorally, mixing goldfish and gouramis comes down to the individual fish. They have very similar behaviors and temperaments. Neither is known for aggression, but some individuals bully others. Both have decorations that may inspire aggressive behavior in other fish. Many goldfish breeds have long, flowing fins or other gadgetry, such as head growths, fleshy bubbles and protruding eyes, that certain fish feel compelled to nip at. Gouramis have modified pelvic fins that look like long antennae, and some goldfish find these irresistible.
Your Tank Setup
If you're determined to mix goldfish and gouramis, consider both the internal and external environment of your tank. Gouramis need finer substrate than goldfish do, but small- to medium-grade aquarium gravel works for both. Both like live plants -- the more, the better. Both are surface breathers and do not need aeration, and both are omnivores, though gouramis may need extra protein.
The challenge comes down to water temperature. If you live in a region where the temperature of your home keeps your tank's water in the 70s year round without resorting to a fish tank heater, you are probably good to give it a try. If this is not the case, you may be risking a temperature gradient that could kill either your gouramis or your goldfish.
Though most species of gourami are hardy, a few can't take poor water quality, ammonia buildup, or pH fluctuations. Don't try these species with goldfish, which are messy and prone to lowering tank water quality.
Angela Libal began writing professionally in 2005. She has published several books, specializing in zoology and animal husbandry. Libal holds a degree in behavioral science: animal science from Moorpark College, a Bachelor of Arts from Sarah Lawrence College and is a graduate student in cryptozoology.