How often should I change my Goldfish water?

I have 2 small goldfish in a 15ltr tank. We have a filter/pump. We feed them 3 pellets each, twice a day. The water, so far is clear - we've had them about 2 weeks. There are only artificial plants, stones and ornaments in the tank.

How often should we change the water? Do we need to change it completely or just 'top it up'?How often should I change my Goldfish water?That's very small for 2 goldfish- they might not live very long. But anyway, you should be doing a partial water change (about 30% of the water) every week.How often should I change my Goldfish water?I suggest scrapping this 15 litre tank.

Get a 100 litre and change water every week about 25%.

Goldfish live long and need big tanks or ponds there produce alot of waste.How often should I change my Goldfish water?Goldfish grow to 12%26quot; or more if well kept. If they live you will have to get a much bigger tank. 50gals. or more.