This 13 February 2019 video says about itself:
How To Keep Your Goldfish Alive For 15 Years
The longest-living goldfish on record was 43 years old. But while they’re supposed to live at least 15 years on average, yours might not have made it to its 5th birthday. The major problem is often toxin accumulation in the tank from fish poop. By seeding the tank with special bacteria, you can break these chemicals down and make the aquarium a safer home for your pet.
Following is a transcript of the video: The longest-living goldfish on record was 43 years old. But chances are your goldfish didn’t even live a quarter that long. In fact, most goldfish don’t survive past their fifth birthday, and that’s a major problem since they’re supposed to live for about as long as your cat. So why are goldfish dying so young? …
But here’s the thing, all the bacteria in the world won’t keep your fish alive if the tank is too small. You see, as these bacteria gobble up goldfish poop, they produce some waste of their own, and if it’s not sufficiently diluted, it can kill your goldfish. A dinky, single-gallon bowl is much too small, and even the 10-gallon tank you see in every pet store won’t cut it. A single adult goldfish needs at least twice that much water to thrive. That’s almost 40 kilograms of water, 2000 times heavier than your goldfish, and even with a huge tank, you’ll still need to replace about 30% of the water every two weeks because besides poop, goldfish release hormones and pheromones into their environment, which, if left unchecked, can stunt their growth.
That might help explain why goldfish in healthy environments can reach the size of an American football, while your last goldfish wasn’t much bigger than your index finger.
Once you have a big, detoxified tank, Goldie just needs one more thing: your attention. Because you’re not swimming in the tank with her, it’s hard to notice if something’s wrong with, say, the water quality. So oftentimes new goldfish owners don’t realize anything’s amiss until it’s too late, and common diseases like fin rot, white spot disease, and fungal infections have already taken hold. So it’s a good idea to pay careful attention to how the water looks and smells, and anyway, spending time with your goldfish will be worth it since you can teach Goldie to swim through hoops and eat from your hand.