I've got a 40-gallon tank with some goldfish I've had for a couple years, and I'm thinking about adding some colorful tropical fish like tetras or guppies. Would they be able to live together, or do they need completely separate setups? I'm worried about temperature differences and whether the goldfish would eat the smaller fish.
Not really a good idea unfortunately. Goldfish and tropical fish have pretty different needs that make them incompatible tank mates. The biggest issue is temperature - goldfish do best in cooler water (around 65-72°F) while tetras and guppies want it warmer (75-80°F). You'd be stuck in the middle trying to keep everyone happy, and nobody thrives that way. Plus goldfish are messy eaters and produce a ton of waste, which means your water quality swings wildly. Tropical fish are usually pickier about that stuff.
The predator thing is real too. Smaller fish like tetras could definitely get eaten, especially if your goldfish get hungry or startled. Even if they're not actively hunting, goldfish will mouth at anything that moves, and they've got that reputation for a reason. Your 40 gallons is actually decent sized, but it's not enough space to keep them far enough apart to prevent problems.
Best move is keeping them separate. Goldfish honestly need their own setup anyway - they get pretty big and live forever (seriously, a couple years is just the start), so a dedicated tank lets them do their thing without stress. Your tropical fish will be way healthier and less stressed without the temperature war and predation risk.
I tried mixing goldfish with some guppies in a 30-gallon once, and it didn't work out. Within a week the goldfish had eaten most of them, and the few survivors were stressed to death. The temperature thing everyone mentions is real too - goldfish want it around 65-72°F, while tropical fish need 75-80°F. That gap matters because cold water stresses tropicals and makes them sick, while warm water makes goldfish lethargic and prone to disease.
Beyond those core issues, there's also the mess factor. Goldfish are pigs - they produce way more waste than tropical fish of similar size. A 40-gallon is borderline for goldfish alone (they really want 20 gallons each), so adding tropicals will just foul the water faster and stress everything. You'd need a much bigger tank plus a heater and way more filtration to even attempt it. Given you've already got goldfish settled in, your best bet is either sticking with them or setting up a separate tropical tank. You'll have better luck keeping either group happy that way.
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