That's a little worrying, but I've heard from a few people that goldfish grow to different sizes depending on their tank and that plenty of people keep goldfish in small tanks. Are you really sure about this?
Yes, we are sure.
Goldfish are one of the most abused species in the hobby. They have been traditionally subjected to poor living conditions, low/non-existent filtration and small tanks/bowls. Most pet goldfish only reach 2-3 inches and live for a few months or even a couple of years. This is so routine and so many people are totally oblivious of the need to research their pets that this is how most goldfish are kept. Most people, including some experienced fishkeepers and pet shop workers, will tell you that goldfish will grow to the size of their tank and don't need much care.
The goldfish lowdown:
Goldfish have a natural lifespan of between 10 and 30 years, depending on variety
Fancy goldfish typically grow to between 6 and 10 inches, depending on variety
Single tailed goldfish (commons, comets, shubunkins, etc) grow to between 10 and 24 inches, depending on variety
Goldfish produce a truly stunning amount of poo and require double the filtration that one would expect
The smallest (6 inch) adult goldfish should be kept in a 3 foot tank of at least 20 gallons, with room for upgrades if it grows
A large goldfish (12 inch) should be kept in a 6 foot tank or pond - larger fish (over 12 inches) need ponds unless you can cope with a 10-12 foot tank.
Fancy goldfish are only suitable for ponds on warm climates or the summer.
Goldfish require filters and most varieties want a high level of flow in the tank to simulate their natural environment
Goldfish need a lot of oxygen in the water and this should be provided by ensuring the surface of the water is always moving
Small tanks and poor water conditions will affect the growth of the fish, from slowing it to stopping it altogether - this is catastropic for the fish and will contribute to an early and painful demise.
Goldfish are social, boisterous fish that should be kept in groups and not housed with other fish if possible
Single tailed goldfish prefer cold temperatures, ideally under 20 C
Fancies like sub-tropical temps of 20-24 C or so
Single tailed goldfish can easily damage the more delicate fancies and shouldn't be housed with them
Goldfish require a diet high in vegetable matter (plants, algae, veggies) but will also appreciate live/frozen foods (such as bloodworms)
Fancy goldfish are prone to constipation (which can cause swimbladder and death) and should be fed sinking goldfish pellets and will benefit from cooked, deshelled and chopped peas every so often.
Goldfish will eat small fish if they can catch them and they will grow big enough to eat danios
A lot of information about a fish that most people think can be put in a bowl.
A 9 gallon tank isn't even really suitable for the danios, tbh, as these little guys get to 2.5 inches, require shoals of at last 6
per species (so you'd need to have 12 danios) and are highly active. They're also renowned fin-nippers and you may find your goldfish's tail gets shorter. A group of 6 danios should have at least 15 imperial gallons and 12 would need at least 20. Contrary to fish shop advice, they are not suitable for small tanks.
I would recommend you either rehome the fish or get a larger tank. The 9 gallon could be used for a micro-community of tropical fish, some small coldwater fish (ask for recommendations) or something more interesting like a betta or pair of dwarf puffers (both tropical).
Welcome to the world of fishkeeping
>nurglespuss
Thanks! It's been a couple of hours and I am happy to report that the resident goldfish has lost interest. If he starts to look too big for his tank, I'll invest in something larger. Same goes for if his fins look as if they've been nipped at.
Thing is, he won't start "to look too big". If he gets stunted, which is very likely in such a small tank, he will stay at a neat 2-4 inches. And die.
He needs to be in a tank big enough for him to grow and he needs it ASAP. He should reach his adult size within two years and be most of the way there after 9-12 months.
I've never understood why people are so keen to wait for the damage to be done (and it is damage - even nipped fins are a wound, however trivial it seems) before they are willing to change something. The number of times I've heard "I'm keeping a male and female betta together - I'll seperate them if they fight" or "I'm keeping chinese algae eaters with fancy goldfish, I'll move them if I see them sucking their slime", etc. How about "I'm keeping a fighting dog around my 2 year old kid, I'll sell it if the kid gets bitten" . . .
