Simply NO

NEVER KEEP GOLDFISH IN A BOWL

Goldfish need a proper tank, 10 gallons for ONE fish is minimum. You also will need good filtration , aeration and live plants, a good variety of quality food.

You need at least a 20 gallon tank for your fish, 20 gallons aren't very expensive And you need to get a good, powerful filter! Get something that will turn at least 5 times your tank volume! I'm using an internal power filter and an external canister filter in my 317L, and they turn about 7 times the tank volume together Combining filtration methods is a great way to deal with the biological load in an aquarium!

It's a waste of time buying a bowl, killing off two-three fish in that and after a while getting a bigger tank. Not to mention that it's a waste of money!! So don't even think about starting off with a smaller tank either. Upgrading is again, waste of money and stressful for the fish.

It is always mentioned on the package of the filter For what sizes of tanks it's rated, and how much water it filters per hour. At least on most better quality filters d:

Actually, mine turns about 8 times the tank volume c: Being a 300L and the filters deal with 2500L/h C:

Otherwise, if you like maths, there is a way to calculate it, but... I wouldn't go there xD

5 goldfish means 50 gallons, okay? (Or 200 liters)

Andrea: Does it say how big tank it's meant for? I should have some formula in one of my books that I could post for you c:

Ilovefish19: Right! You need to know about cycling, proper regular maintenance, feeding and WATER TESTING D:

ILovefish19, actually they don't "eat" their own poo, but in a small tank and not enough filtration, etc, like small tanksize they soon will swim in their own sewerage and poison themselves with all those nitrogens. They will suffer a horrible death.

So as already mentioned: 5 fish = 50 gallons

Andrea, what does it say on the tank package? Or is there a label maybe on the impeller or so?

Will do Nossie And Hanna, the only paper the tank/filter came with was how to make the stand and it was mainly in chinese haha. Lucky my boyfriend knew what to do. I'll have a look on the bit that is actually in the water tomorrow. It might say. It's like an aquaone tank, you know how they come with the filter already? Well do they normally say the rate?

Water testing is simple yet alot of people don't do it so that's why we are checking as it is very important

A powerful filter would be great but you still need to water changes at least once a fortnight (with a really good filter). Your fish will love it and though filters get rid of muck, they don't actually make the water fresh. So make sure you do regular ones

Filters don't remove the nitrates, they will be building up slowly even with a powerful filter system. In other words, water changes are CRUCIAL to keep the water quality acceptable. A 50 gallon tank will be quite big, so you have a good "safety buffer" in that case, since bigger tank means more water, which in turn means more stable temperature and water parameters.
But don't ever think that you won't need to change water any less than every second week.