test your water every couple of days when the nitrate reading hits forty do a 25% water change. recheck your water every few days until the water test comes in at 40 do another 25% water change. if you have to do 2 water changes a week thats what u have to do. as your fish grow the nitrates will rise quicker so larger or more frequent changes will be needed. if this becomes to much work either get a bigger tank or reduce the mumber of fish you have.

if your ammonia or nitrites start to rise above 0 you'll need more biological filtration.

necromancer has got it spot on. If your nitrates are above 40 before a water change then you will have to do more/bigger water changes. When/if the water changes become too much effort for you then its time to rehome a fish.

You also have to think about what happens if you go on holiday, you can reduce feeding but you may still have to organize for someone to do a water change, with less fish it can be left longer before the nitrates get too high