You will probably have to upgrade the filter and your tank as your fish grow.
If a filter is rated for a certain size tank it is usually too small for that size tank, that's the general rule. A filter that is rated for twice your tank size is usually adequate.
6 goldfish will produce enough ammonia to overpower your filter and even if your biobacteria are keeping up, enough ammonia will be turned to nitrates to effect the health of your goldfish.
Keep testing, and if you see your nitrates getting too high (books usually recommend keeping it below 40), consider upgrading to a larger tank and/or doing more water changes.
If you see ammonia show up at all your filter isn't keeping up with the load so you'll need more adequate filtration. While your filter may be adequate now, having too small an amount of biobacteria means your biological filtration can be more easily disrupted, so it's always good to have as much filtration and biomedia as possible.